@Override
    public final boolean onPostMessage(String message, Bundle extras) {
        return postMessage(message, extras);
    }
Notifies the client that the postMessage channel requested with
{@link CustomTabsService#requestPostMessageChannel(
CustomTabsSessionToken, android.net.Uri)} is ready. This method should be
called when the browser binds to the client side {@link PostMessageService} and also readies
a connection to the web frame.

@param extras Reserved for future use.
@return Whether the notification was sent to the remote successfully.
customtabs/src/android/support/customtabs/PostMessageServiceConnection.java