[aw] Provide a Handler on which platform service tasks can execute

Bug: 897266
Change-Id: Icc0f7b1671031f76e2781ed2e6e217c50f0925e6
Reviewed-on: https://chromium-review.googlesource.com/c/1292469Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: Tobias Sargeant <tobiasjs@chromium.org>
Commit-Queue: Tobias Sargeant <tobiasjs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601676}

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

@@ -5,6 +5,8 @@
 package org.chromium.android_webview;
 
 import android.content.Context;
+import android.os.Handler;
+import android.os.HandlerThread;
 import android.support.annotation.NonNull;
 
 import org.chromium.base.Callback;
@@ -16,9 +18,14 @@ import org.chromium.base.ThreadUtils;
  */
 public abstract class PlatformServiceBridge {
     private static final String TAG = "PlatformServiceBrid-";
+
     private static PlatformServiceBridge sInstance;
     private static final Object sInstanceLock = new Object();
 
+    private static HandlerThread sHandlerThread;
+    private static Handler sHandler;
+    private static final Object sHandlerLock = new Object();
+
     protected PlatformServiceBridge() {}
 
     @SuppressWarnings("unused")
@@ -43,6 +50,18 @@ public abstract class PlatformServiceBridge {
         }
     }
 
+    // Return a handler appropriate for executing blocking Platform Service tasks.
+    public static Handler getHandler() {
+        synchronized (sHandlerLock) {
+            if (sHandler == null) {
+                sHandlerThread = new HandlerThread("PlatformServiceBridgeHandlerThread");
+                sHandlerThread.start();
+                sHandler = new Handler(sHandlerThread.getLooper());
+            }
+        }
+        return sHandler;
+    }
+
     // Can WebView use Google Play Services (a.k.a. GMS)?
     public boolean canUseGms() {
         return false;