Add GMS to WebView build

This grows standalone WebView APKs by ~1MB. Ameliorating the growth is
tracked in the same bug.

BUG=662166

Review-Url: https://codereview.chromium.org/2541993002
Cr-Commit-Position: refs/heads/master@{#436097}

android_webview/BUILD.gn

@@ -622,6 +622,8 @@ android_library("android_webview_java") {
     "//net/android:net_java",
     "//third_party/android_tools:android_support_annotations_java",
     "//ui/android:ui_java",
+    google_play_services_library,
+    google_play_services_resources,
   ]
 
   if (enable_configuration_policy) {

android_webview/apk/java/proguard.flags

@@ -63,3 +63,6 @@
   public void setDisabledActionModeMenuItems(int);
   public int getDisabledActionModeMenuItems();
 }
+
+# We strip some unused resources when preprocessing the GMS client libs.
+-dontwarn com.google.android.gms.R**

android_webview/java/src/org/chromium/android_webview/PlatformServiceBridge.java

@@ -49,5 +49,13 @@ public class PlatformServiceBridge {
         return sInstance;
     }
 
+    // Try to enable WebView to use Google Play Services (a.k.a. GMS) APIs. Return true on success.
+    // Do not use GMS APIs before this has returned true, or if it returns false. This can be called
+    // from multiple threads, so long as no thread uses GMS APIs before at least one call has
+    // returned true. (The easy way is for each thread to wait for its own call to return true.)
+    public boolean tryEnableGms() {
+        return false;
+    }
+
     public void setMetricsSettingListener(ValueCallback<Boolean> callback) {}
 }