Add a service to handle postMessage callbacks to client

customtabs/src/android/support/customtabs/CustomTabsSession.java

Index: customtabs/src/android/support/customtabs/CustomTabsSession.java
diff --git a/customtabs/src/android/support/customtabs/CustomTabsSession.java b/customtabs/src/android/support/customtabs/CustomTabsSession.java
index 8157733298b700d7a0cb65e82d6884b2159460bb..acf8f921807bdb14c71e8b84f20e75bd33f1a1eb 100644
--- a/customtabs/src/android/support/customtabs/CustomTabsSession.java
+++ b/customtabs/src/android/support/customtabs/CustomTabsSession.java
@@ -36,6 +36,7 @@ import java.util.List;
  */
 public final class CustomTabsSession {
     private static final String TAG = "CustomTabsSession";
+    private final Object mLock = new Object();
     private final ICustomTabsService mService;
     private final ICustomTabsCallback mCallback;
     private final ComponentName mComponentName;
@@ -151,20 +152,43 @@ public final class CustomTabsSession {
         }
     }
 
-    public boolean validatePostMessageOrigin() {
+    /**
+     * Sends a request to create a two way postMessage channel between the client and the browser.
+     *
+     * @param postMessageOrigin      A origin that the client is requesting to be identified as
+     *                               during the postMessage communication.
+     * @return Whether the implementation accepted the request. Note that returning true
+     *         here doesn't mean an origin has already been assigned as the validation is
+     *         asynchronous.
+     */
+    public boolean requestPostMessageChannel(Uri postMessageOrigin) {
         try {
-            return mService.validatePostMessageOrigin(mCallback);
+            return mService.requestPostMessageChannel(
+                    mCallback, postMessageOrigin);
         } catch (RemoteException e) {
             return false;
         }
     }
 
+    /**
+     * Sends a postMessage request using the origin communicated via
+     * {@link CustomTabsService#requestPostMessageChannel(
+     * CustomTabsSessionToken, Uri)}. Fails when called before
+     * {@link PostMessageServiceConnection#notifyMessageChannelReady(Bundle)} is received on
+     * the client side.
+     *
+     * @param message The message that is being sent.
+     * @param extras Reserved for future use.
+     * @return An integer constant about the postMessage request result.

[Benoit L, 2017/01/25 21:25:03]

nit: Perhaps specify where are these constants? (The Result "enum")

[Yusuf, 2017/01/25 22:02:00]

The most viable way to refer to the enum was to refer to one of the constants.

+     */
     @Result
-    public synchronized int postMessage(String message, Bundle extras) {
-        try {
-            return mService.postMessage(mCallback, message, extras);
-        } catch (RemoteException e) {
-            return CustomTabsService.RESULT_FAILURE_REMOTE_ERROR;
+    public int postMessage(String message, Bundle extras) {
+        synchronized (mLock) {
+            try {
+                return mService.postMessage(mCallback, message, extras);
+            } catch (RemoteException e) {
+                return CustomTabsService.RESULT_FAILURE_REMOTE_ERROR;
+            }
         }
     }