@Result
    public int postMessage(String message, Bundle extras) {
        addIdToBundle(extras);
        synchronized (mLock) {
            try {
                return mService.postMessage(mCallback, message, extras);
            } catch (RemoteException e) {
                return CustomTabsService.RESULT_FAILURE_REMOTE_ERROR;
            }
        }
    }
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. Will return
       {@link CustomTabsService#RESULT_SUCCESS} if successful.
customtabs/src/android/support/customtabs/CustomTabsSession.java