WebLayer: hook up MediaSession.

This adds the glue code for MediaSession to work in WebLayer.

A new Service class is required in the client library to display
the notification. This service proxies lifetime events to WebLayer
implementation, which then glues together componentized code.

Of note:
1. notifications displayed with Service#startForeground cannot have
   a tag, so the ID must be unique across all application notifications.
   There's a default ID for the MediaSession notification, but an API
   is also added to WebLayer to allow setting a different ID, in case
   this one conflicts.
2. favicons are not supported (normally they'd be used as fallbacks for
   iconography/images not provided by the system)
3. Extra handling is required for the action button icons to work,
   see WebLayerNotificationBuilder. On L, we fall back to system icons,
   which look bad because of their low resolution, but are at least
   functional.

      https://googlechrome.github.io/samples/media-session/

Test: automated tests TODO; manual testing on
Bug: 1066263
Change-Id: I5e2ae13a07315e73aae474251cb5c0424804a427
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2220551Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Clark DuVall <cduvall@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782054}

weblayer/app/content_main_delegate_impl.cc

@@ -139,10 +139,6 @@ bool ContentMainDelegateImpl::BasicStartupComplete(int* exit_code) {
   cl->AppendSwitch(::switches::kDisablePresentationAPI);
   // TODO(crbug.com/1057100): make remote-playback-api work with WebLayer.
   cl->AppendSwitch(::switches::kDisableRemotePlaybackAPI);
-#if defined(OS_ANDROID)
-  // TODO(crbug.com/1066263): make MediaSession work with WebLayer.
-  cl->AppendSwitch(::switches::kDisableMediaSessionAPI);
-#endif
   DisableFeaturesIfNotSet({
     // TODO(crbug.com/1025619): make web-payments work with WebLayer.
     ::features::kWebPayments,

weblayer/browser/browser_main_parts_impl.cc

@@ -18,7 +18,6 @@
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/devtools_agent_host.h"
 #include "content/public/browser/render_process_host.h"
-#include "content/public/common/content_switches.h"
 #include "content/public/common/main_function_params.h"
 #include "content/public/common/url_constants.h"
 #include "services/service_manager/embedder/result_codes.h"
@@ -36,16 +35,19 @@
 #include "weblayer/public/main.h"
 
 #if defined(OS_ANDROID)
+#include "base/command_line.h"
 #include "components/crash/content/browser/child_exit_observer_android.h"
 #include "components/crash/content/browser/child_process_crash_observer_android.h"
 #include "components/crash/core/common/crash_key.h"
 #include "components/javascript_dialogs/android/app_modal_dialog_view_android.h"  // nogncheck
 #include "components/javascript_dialogs/app_modal_dialog_manager.h"  // nogncheck
 #include "content/public/browser/web_contents.h"
+#include "content/public/common/content_switches.h"
 #include "net/android/network_change_notifier_factory_android.h"
 #include "net/base/network_change_notifier.h"
 #include "weblayer/browser/android/metrics/uma_utils.h"
 #include "weblayer/browser/java/jni/MojoInterfaceRegistrar_jni.h"
+#include "weblayer/browser/weblayer_factory_impl_android.h"
 #endif
 
 #if defined(USE_X11)
@@ -112,6 +114,15 @@ int BrowserMainPartsImpl::PreCreateThreads() {
       std::make_unique<crash_reporter::ChildProcessCrashObserver>());
 
   crash_reporter::InitializeCrashKeys();
+
+  // MediaSession was implemented in M85, and requires both implementation and
+  // client libraries to be at least that new. The version check has to be in
+  // the browser process, but the command line flag is automatically propagated
+  // to renderers.
+  if (WebLayerFactoryImplAndroid::GetClientMajorVersion() < 85) {
+    base::CommandLine::ForCurrentProcess()->AppendSwitch(
+        ::switches::kDisableMediaSessionAPI);
+  }
 #endif
 
   return service_manager::RESULT_CODE_NORMAL_EXIT;

weblayer/browser/java/BUILD.gn

@@ -26,6 +26,7 @@ android_resources("weblayer_resources") {
     ":weblayer_strings_grd",
     "//components/blocked_content/android:java_resources",
     "//components/browser_ui/http_auth/android:java_resources",
+    "//components/browser_ui/media/android:java_resources",
     "//components/browser_ui/settings/android:java_resources",
     "//components/browser_ui/site_settings/android:java_resources",
     "//components/browser_ui/strings/android:browser_ui_strings_grd",
@@ -104,6 +105,7 @@ android_library("java") {
     "org/chromium/weblayer_private/IntentUtils.java",
     "org/chromium/weblayer_private/InterceptNavigationDelegateClientImpl.java",
     "org/chromium/weblayer_private/LocaleChangedBroadcastReceiver.java",
+    "org/chromium/weblayer_private/MediaSessionManager.java",
     "org/chromium/weblayer_private/MediaStreamManager.java",
     "org/chromium/weblayer_private/MojoInterfaceRegistrar.java",
     "org/chromium/weblayer_private/NavigationControllerImpl.java",
@@ -151,6 +153,7 @@ android_library("java") {
     "//components/autofill/android/provider:java",
     "//components/browser_ui/client_certificate/android:java",
     "//components/browser_ui/http_auth/android:java",
+    "//components/browser_ui/media/android:java",
     "//components/browser_ui/modaldialog/android:java",
     "//components/browser_ui/notifications/android:java",
     "//components/browser_ui/settings/android:java",

weblayer/browser/java/org/chromium/weblayer_private/MediaSessionManager.java

+// Copyright 2020 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.weblayer_private;
+
+import android.annotation.SuppressLint;
+import android.app.Service;
+import android.content.Intent;
+import android.support.v4.media.session.MediaSessionCompat;
+
+import org.chromium.components.browser_ui.media.MediaNotificationController;
+import org.chromium.components.browser_ui.media.MediaNotificationInfo;
+import org.chromium.components.browser_ui.media.MediaSessionHelper;
+import org.chromium.components.browser_ui.notifications.ChromeNotification;
+import org.chromium.components.browser_ui.notifications.ChromeNotificationBuilder;
+import org.chromium.components.browser_ui.notifications.ForegroundServiceUtils;
+import org.chromium.components.browser_ui.notifications.NotificationMetadata;
+
+/**
+ * A glue class for MediaSession.
+ * This class defines delegates that provide WebLayer-specific behavior to shared MediaSession code.
+ * It also manages the lifetime of {@link MediaNotificationController} and the {@link Service}
+ * associated with the notification.
+ */
+class MediaSessionManager {
+    // This is a singleton because there's only at most one MediaSession active at a time.
+    @SuppressLint("StaticFieldLeak")
+    static MediaNotificationController sController;
+
+    private static int sNotificationId = 0;
+
+    static void serviceStarted(Service service, Intent intent) {
+        if (sController != null && sController.processIntent(service, intent)) return;
+
+        // The service has been started with startForegroundService() but the
+        // notification hasn't been shown. See similar logic in {@link
+        // ChromeMediaNotificationControllerDelegate}.
+        MediaNotificationController.finishStartingForegroundServiceOnO(
+                service, createChromeNotificationBuilder().buildChromeNotification());
+        // Call stopForeground to guarantee Android unset the foreground bit.
+        ForegroundServiceUtils.getInstance().stopForeground(
+                service, Service.STOP_FOREGROUND_REMOVE);
+        service.stopSelf();
+    }
+
+    static void serviceDestroyed() {
+        if (sController != null) sController.onServiceDestroyed();
+        sController = null;
+    }
+
+    static MediaSessionHelper.Delegate createMediaSessionHelperDelegate(int tabId) {
+        return new MediaSessionHelper.Delegate() {
+            @Override
+            public Intent createBringTabToFrontIntent() {
+                return IntentUtils.createBringTabToFrontIntent(tabId);
+            }
+
+            @Override
+            public boolean fetchLargeFaviconImage() {
+                // TODO(crbug.com/1076463): WebLayer doesn't support favicons.
+                return false;
+            }
+
+            @Override
+            public MediaNotificationInfo.Builder createMediaNotificationInfoBuilder() {
+                ensureNotificationId();
+                return new MediaNotificationInfo.Builder().setInstanceId(tabId).setId(
+                        sNotificationId);
+            }
+
+            @Override
+            public void showMediaNotification(MediaNotificationInfo notificationInfo) {
+                assert notificationInfo.id == sNotificationId;
+                if (sController == null) {
+                    sController = new MediaNotificationController(
+                            new WebLayerMediaNotificationControllerDelegate());
+                }
+                sController.mThrottler.queueNotification(notificationInfo);
+            }
+
+            @Override
+            public void hideMediaNotification() {
+                if (sController != null) sController.hideNotification(tabId);
+            }
+
+            @Override
+            public void activateAndroidMediaSession() {
+                if (sController != null) sController.activateAndroidMediaSession(tabId);
+            }
+        };
+    }
+
+    private static class WebLayerMediaNotificationControllerDelegate
+            implements MediaNotificationController.Delegate {
+        @Override
+        public Intent createServiceIntent() {
+            return WebLayerImpl.createMediaSessionServiceIntent();
+        }
+
+        @Override
+        public String getAppName() {
+            return WebLayerImpl.getClientApplicationName();
+        }
+
+        @Override
+        public String getNotificationGroupName() {
+            return "org.chromium.weblayer.MediaSession";
+        }
+
+        @Override
+        public ChromeNotificationBuilder createChromeNotificationBuilder() {
+            return MediaSessionManager.createChromeNotificationBuilder();
+        }
+
+        @Override
+        public void onMediaSessionUpdated(MediaSessionCompat session) {
+            // This is only relevant when casting.
+        }
+
+        @Override
+        public void logNotificationShown(ChromeNotification notification) {}
+    }
+
+    private static ChromeNotificationBuilder createChromeNotificationBuilder() {
+        ensureNotificationId();
+
+        // Only the null tag will work as expected, because {@link Service#startForeground()} only
+        // takes an ID and no tag. If we pass a tag here, then the notification that's used to
+        // display a paused state (no foreground service) will not be identified as the same one
+        // that's used with the foreground service.
+        return WebLayerNotificationBuilder.create(
+                WebLayerNotificationChannels.ChannelId.MEDIA_PLAYBACK,
+                new NotificationMetadata(0, null /*notificationTag*/, sNotificationId));
+    }
+
+    private static void ensureNotificationId() {
+        if (sNotificationId == 0) sNotificationId = WebLayerImpl.getMediaSessionNotificationId();
+    }
+}

weblayer/browser/java/org/chromium/weblayer_private/TabImpl.java

@@ -27,6 +27,7 @@ import org.chromium.base.annotations.NativeMethods;
 import org.chromium.components.autofill.AutofillActionModeCallback;
 import org.chromium.components.autofill.AutofillProvider;
 import org.chromium.components.browser_ui.http_auth.LoginPrompt;
+import org.chromium.components.browser_ui.media.MediaSessionHelper;
 import org.chromium.components.browser_ui.util.BrowserControlsVisibilityDelegate;
 import org.chromium.components.browser_ui.util.ComposedBrowserControlsVisibilityDelegate;
 import org.chromium.components.embedder_support.contextmenu.ContextMenuParams;
@@ -112,6 +113,7 @@ public final class TabImpl extends ITab.Stub implements LoginPrompt.Observer {
     private InterceptNavigationDelegateClientImpl mInterceptNavigationDelegateClient;
     private InterceptNavigationDelegateImpl mInterceptNavigationDelegate;
     private InfoBarContainer mInfoBarContainer;
+    private MediaSessionHelper mMediaSessionHelper;
 
     private boolean mPostContainerViewInitDone;
 
@@ -251,6 +253,14 @@ public final class TabImpl extends ITab.Stub implements LoginPrompt.Observer {
         };
         // addObserver() calls to observer when added.
         WebLayerAccessibilityUtil.get().addObserver(mAccessibilityObserver);
+
+        // MediaSession only works if the client is new enough. Sadly, passing
+        // kDisableMediaSessionAPI does not fully disable the API, so a check is also necessary
+        // before installing this observer.
+        if (WebLayerFactoryImpl.getClientMajorVersion() >= 85) {
+            mMediaSessionHelper = new MediaSessionHelper(
+                    mWebContents, MediaSessionManager.createMediaSessionHelperDelegate(mId));
+        }
     }
 
     private void doInitAfterSettingContainerView() {

weblayer/browser/java/org/chromium/weblayer_private/WebLayerImpl.java

@@ -4,6 +4,7 @@
 
 package org.chromium.weblayer_private;
 
+import android.app.Service;
 import android.content.Context;
 import android.content.Intent;
 import android.content.SharedPreferences;
@@ -331,6 +332,19 @@ public final class WebLayerImpl extends IWebLayer.Stub {
         }
     }
 
+    @Override
+    public void onMediaSessionServiceStarted(IObjectWrapper sessionService, Intent intent) {
+        StrictModeWorkaround.apply();
+        MediaSessionManager.serviceStarted(
+                ObjectWrapper.unwrap(sessionService, Service.class), intent);
+    }
+
+    @Override
+    public void onMediaSessionServiceDestroyed() {
+        StrictModeWorkaround.apply();
+        MediaSessionManager.serviceDestroyed();
+    }
+
     @Override
     public void enumerateAllProfileNames(IObjectWrapper valueCallback) {
         StrictModeWorkaround.apply();
@@ -383,6 +397,30 @@ public final class WebLayerImpl extends IWebLayer.Stub {
         }
     }
 
+    public static Intent createMediaSessionServiceIntent() {
+        if (sClient == null) {
+            throw new IllegalStateException("WebLayer should have been initialized already.");
+        }
+
+        try {
+            return sClient.createMediaSessionServiceIntent();
+        } catch (RemoteException e) {
+            throw new APICallException(e);
+        }
+    }
+
+    public static int getMediaSessionNotificationId() {
+        if (sClient == null) {
+            throw new IllegalStateException("WebLayer should have been initialized already.");
+        }
+
+        try {
+            return sClient.getMediaSessionNotificationId();
+        } catch (RemoteException e) {
+            throw new APICallException(e);
+        }
+    }
+
     public static String getClientApplicationName() {
         Context context = ContextUtils.getApplicationContext();
         return new StringBuilder()

weblayer/browser/java/org/chromium/weblayer_private/WebLayerNotificationBuilder.java

@@ -4,6 +4,9 @@
 
 package org.chromium.weblayer_private;
 
+import android.annotation.TargetApi;
+import android.app.Notification;
+import android.app.PendingIntent;
 import android.content.Context;
 import android.graphics.drawable.Icon;
 import android.os.Build;
@@ -40,17 +43,67 @@ final class WebLayerNotificationBuilder extends NotificationBuilder {
     public ChromeNotificationBuilder setSmallIcon(int icon) {
         if (WebLayerImpl.isAndroidResource(icon)) {
             super.setSmallIcon(icon);
-            return this;
+        } else if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
+            super.setSmallIcon(createIcon(icon));
+        } else {
+            // Some fallback is required, or the notification won't appear.
+            super.setSmallIcon(getFallbackAndroidResource(icon));
         }
+        return this;
+    }
 
-        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
-            super.setSmallIcon(
-                    Icon.createWithResource(WebViewFactory.getLoadedPackageInfo().packageName,
-                            WebLayerImpl.getResourceIdForSystemUi(icon)));
+    @Override
+    @SuppressWarnings("deprecation")
+    public ChromeNotificationBuilder addAction(int icon, CharSequence title, PendingIntent intent) {
+        if (WebLayerImpl.isAndroidResource(icon)) {
+            super.addAction(icon, title, intent);
+        } else if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
+            super.addAction(
+                    new Notification.Action.Builder(createIcon(icon), title, intent).build());
         } else {
-            // Some fallback is required, or the notification won't appear.
-            super.setSmallIcon(android.R.drawable.radiobutton_on_background);
+            super.addAction(getFallbackAndroidResource(icon), title, intent);
         }
         return this;
     }
+
+    @TargetApi(Build.VERSION_CODES.M)
+    private Icon createIcon(int resId) {
+        return Icon.createWithResource(WebViewFactory.getLoadedPackageInfo().packageName,
+                WebLayerImpl.getResourceIdForSystemUi(resId));
+    }
+
+    /**
+     * Finds a reasonable replacement for the given app-defined resource from among stock android
+     * resources. This is useful when {@link Icon} is not available.
+     */
+    private int getFallbackAndroidResource(int appResourceId) {
+        if (appResourceId == R.drawable.ic_play_arrow_white_36dp) {
+            return android.R.drawable.ic_media_play;
+        }
+        if (appResourceId == R.drawable.ic_pause_white_36dp) {
+            return android.R.drawable.ic_media_pause;
+        }
+        if (appResourceId == R.drawable.ic_stop_white_36dp) {
+            // There's no ic_media_stop. This standin is at least a square. In practice this
+            // shouldn't ever come up as stop is only used in (Chrome) cast notifications.
+            return android.R.drawable.checkbox_off_background;
+        }
+        if (appResourceId == R.drawable.ic_skip_previous_white_36dp) {
+            return android.R.drawable.ic_media_previous;
+        }
+        if (appResourceId == R.drawable.ic_skip_next_white_36dp) {
+            return android.R.drawable.ic_media_next;
+        }
+        if (appResourceId == R.drawable.ic_fast_forward_white_36dp) {
+            return android.R.drawable.ic_media_ff;
+        }
+        if (appResourceId == R.drawable.ic_fast_rewind_white_36dp) {
+            return android.R.drawable.ic_media_rew;
+        }
+        if (appResourceId == R.drawable.audio_playing) {
+            return android.R.drawable.ic_lock_silent_mode_off;
+        }
+
+        return android.R.drawable.radiobutton_on_background;
+    }
 }

weblayer/browser/java/org/chromium/weblayer_private/interfaces/IWebLayer.aidl

@@ -96,4 +96,8 @@ interface IWebLayer {
   ISiteSettingsFragment createSiteSettingsFragmentImpl(
       in IRemoteFragmentClient remoteFragmentClient,
       in IObjectWrapper fragmentArgs) = 16;
+
+  // Added in Version 85.
+  void onMediaSessionServiceStarted(in IObjectWrapper sessionService, in Intent intent) = 17;
+  void onMediaSessionServiceDestroyed() = 18;
 }

weblayer/browser/java/org/chromium/weblayer_private/interfaces/IWebLayerClient.aidl

@@ -8,4 +8,6 @@ import android.content.Intent;
 
 interface IWebLayerClient {
   Intent createIntent() = 0;
+  Intent createMediaSessionServiceIntent() = 1;
+  int getMediaSessionNotificationId() = 2;
 }

weblayer/public/java/AndroidManifest.xml

@@ -13,6 +13,7 @@
     <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION"/>
     <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
     <uses-permission android:name="android.permission.CAMERA"/>
+    <uses-permission android:name="android.permission.FOREGROUND_SERVICE" />
     <uses-permission android:name="android.permission.INTERNET"/>
     <uses-permission android:name="android.permission.MODIFY_AUDIO_SETTINGS"/>
     <uses-permission android:name="android.permission.RECORD_AUDIO"/>
@@ -75,5 +76,12 @@
                 <action android:name="org.chromium.weblayer.webrtc.ACTIVATE_TAB"/>
             </intent-filter>
         </receiver>
+
+        <service android:name="org.chromium.weblayer.MediaSessionService"
+            android:exported="false">
+            <intent-filter>
+                <action android:name="android.intent.action.MEDIA_BUTTON" />
+            </intent-filter>
+        </service>
     </application>
 </manifest>

weblayer/public/java/BUILD.gn

@@ -18,12 +18,14 @@ jinja_template("weblayer_client_manifest") {
 }
 
 android_resources("client_resources") {
+  custom_package = "org.chromium.weblayer"
   sources = [
     "res/values-night/colors.xml",
     "res/values-night/values.xml",
     "res/values-v27/styles.xml",
     "res/values-v28/styles.xml",
     "res/values/colors.xml",
+    "res/values/ids.xml",
     "res/values/styles.xml",
     "res/values/values.xml",
     "res/xml/weblayer_file_paths.xml",
@@ -58,6 +60,7 @@ android_library("java") {
     "org/chromium/weblayer/LoadError.java",
     "org/chromium/weblayer/MediaCaptureCallback.java",
     "org/chromium/weblayer/MediaCaptureController.java",
+    "org/chromium/weblayer/MediaSessionService.java",
     "org/chromium/weblayer/NavigateParams.java",
     "org/chromium/weblayer/Navigation.java",
     "org/chromium/weblayer/NavigationCallback.java",

weblayer/public/java/org/chromium/weblayer/MediaSessionService.java

+// Copyright 2020 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.weblayer;
+
+import android.app.Service;
+import android.content.Context;
+import android.content.Intent;
+import android.content.IntentFilter;
+import android.media.AudioManager;
+import android.os.IBinder;
+import android.os.RemoteException;
+
+import org.chromium.weblayer_private.interfaces.ObjectWrapper;
+
+/**
+ * A foreground {@link Service} for the Web MediaSession API.
+ * This class is a thin wrapper that forwards lifecycle events to the WebLayer implementation, which
+ * in turn manages a system notification and {@link MediaSession}. This service will be in the
+ * foreground when the MediaSession is active.
+ * @since 85
+ */
+public class MediaSessionService extends Service {
+    // A helper to automatically pause the media session when a user removes headphones.
+    private BroadcastReceiver mAudioBecomingNoisyReceiver;
+
+    public MediaSessionService() {
+        if (WebLayer.getSupportedMajorVersionInternal() < 85) {
+            throw new UnsupportedOperationException();
+        }
+    }
+
+    @Override
+    public IBinder onBind(Intent intent) {
+        return null;
+    }
+
+    @Override
+    public void onCreate() {
+        super.onCreate();
+
+        mAudioBecomingNoisyReceiver = new BroadcastReceiver() {
+            @Override
+            public void onReceive(Context context, Intent intent) {
+                if (!AudioManager.ACTION_AUDIO_BECOMING_NOISY.equals(intent.getAction())) {
+                    return;
+                }
+
+                Intent i = new Intent(getApplication(), MediaSessionService.class);
+                i.setAction(intent.getAction());
+                getApplication().startService(i);
+            }
+        };
+
+        IntentFilter filter = new IntentFilter(AudioManager.ACTION_AUDIO_BECOMING_NOISY);
+        registerReceiver(mAudioBecomingNoisyReceiver, filter);
+    }
+
+    @Override
+    public void onDestroy() {
+        super.onDestroy();
+
+        try {
+            getWebLayer().getImpl().onMediaSessionServiceDestroyed();
+        } catch (RemoteException e) {
+            throw new RuntimeException(e);
+        }
+
+        unregisterReceiver(mAudioBecomingNoisyReceiver);
+    }
+
+    @Override
+    public int onStartCommand(Intent intent, int flags, int startId) {
+        try {
+            getWebLayer().getImpl().onMediaSessionServiceStarted(ObjectWrapper.wrap(this), intent);
+        } catch (RemoteException e) {
+            throw new RuntimeException(e);
+        }
+
+        return START_NOT_STICKY;
+    }
+
+    private WebLayer getWebLayer() {
+        WebLayer webLayer;
+        try {
+            webLayer = WebLayer.getLoadedWebLayer(getApplication());
+        } catch (UnsupportedVersionException e) {
+            throw new RuntimeException(e);
+        }
+        if (webLayer == null) {
+            throw new IllegalStateException("WebLayer not initialized");
+        }
+        return webLayer;
+    }
+}

weblayer/public/java/org/chromium/weblayer/WebLayer.java

@@ -180,6 +180,14 @@ public class WebLayer {
         return mImpl;
     }
 
+    static WebLayer getLoadedWebLayer(@NonNull Context appContext)
+            throws UnsupportedVersionException {
+        ThreadCheck.ensureOnUiThread();
+        appContext = appContext.getApplicationContext();
+        checkAvailable(appContext);
+        return getWebLayerLoader(appContext).getLoadedWebLayer();
+    }
+
     /**
      * Returns the supported version. Using any functions defined in a newer version than
      * returned by {@link getSupportedMajorVersion} result in throwing an
@@ -364,6 +372,10 @@ public class WebLayer {
             }
         }
 
+        WebLayer getLoadedWebLayer() {
+            return mWebLayer;
+        }
+
         @Nullable
         private IWebLayer getIWebLayer(@NonNull Context appContext) {
             if (mIWebLayer != null) return mIWebLayer;
@@ -672,5 +684,18 @@ public class WebLayer {
             // client library because it's referenced in the manifest.
             return new Intent(WebLayer.getAppContext(), BroadcastReceiver.class);
         }
+
+        @Override
+        public Intent createMediaSessionServiceIntent() {
+            StrictModeWorkaround.apply();
+            return new Intent(WebLayer.getAppContext(), MediaSessionService.class);
+        }
+
+        @Override
+        public int getMediaSessionNotificationId() {
+            StrictModeWorkaround.apply();
+            // The id is part of the public library to avoid conflicts.
+            return R.id.weblayer_media_session_notification;
+        }
     }
 }

weblayer/public/java/res/values/ids.xml

+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright 2020 The Chromium Authors. All rights reserved.
+     Use of this source code is governed by a BSD-style license that can be
+     found in the LICENSE file. -->
+
+<resources>
+    <item type="id" name="weblayer_media_session_notification" />
+</resources>