Add postMessage APIs to the support lib

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 290150db1837a46d9a91c9aae52396cbafaf3cf7..e7a0a53f944a075700c7e5f89ddfe40eae3de4e6 100644
--- a/customtabs/src/android/support/customtabs/CustomTabsSession.java
+++ b/customtabs/src/android/support/customtabs/CustomTabsSession.java
@@ -150,6 +150,43 @@ public final class CustomTabsSession {
         }
     }
 
+    /**
+     * Sends a request to the implementation for validate and assign a postMessage origin for the

[Benoit L, 2016/11/07 17:50:38]

nit: s/for/to/ ?
(not a native speaker, so I'm not 100% sure)

[Yusuf, 2016/11/07 18:56:42]

Done.

+     * related {@link CustomTabsSession}.
+     *
+     * <p>This also acts a trigger to setup a postMessage communication channel.
+     *
+     * @return Whether the implementation accepted the validation request. Note that returning true
+     *         here doesn't mean an origin has already been assigned as the validation can be
+     *         asynchronous.
+     */
+    public boolean validatePostMessageOrigin() {
+        try {
+            return mService.validatePostMessageOrigin(mCallback);
+        } catch (RemoteException e) {
+            return false;
+        }
+    }
+
+    /**
+     * Sends a postMessage request using the origin that has been validated and communicated via
+     * {@link CustomTabsCallback#onMessageChannelReady(Uri, Bundle)}. This method is syncronized

[Benoit L, 2016/11/07 17:50:38]

nit: synchronized

[Yusuf, 2016/11/07 18:56:42]

Done.

+     * to preserve the ordering of postMessage requests as they are going through the binder thread

[Benoit L, 2016/11/07 17:50:38]

Also, I'm not sure that we need to explain why a given method is synchronized in
the javadoc. What matters to app developers are the threading guarantees, not
the actual implementation.

Basically, the guarantee we provide is:
"If postMessage() is called from a single thread, then the messages will be
posted in the same order to the web page.".

[Yusuf, 2016/11/07 18:56:42]

Done.

+     * pool. Returns false when called without a preceding
+     * {@link CustomTabsService#validatePostMessageOrigin(CustomTabsSessionToken)}.
+     *
+     * @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, 2016/11/07 17:50:38]

ditto, @IntDef

[Yusuf, 2016/11/07 18:56:42]

Done.

+     */
+    public synchronized int postMessage(String message, Bundle extras) {
+        try {
+            return mService.postMessage(mCallback, message, extras);
+        } catch (RemoteException e) {
+            return CustomTabsService.RESULT_FAILURE_REMOTE_ERROR;
+        }
+    }
+
     /* package */ IBinder getBinder() {
         return mCallback.asBinder();
     }