public final boolean postMessage(String message, Bundle extras) {
        if (!isBoundToService()) return false;
        synchronized (mLock) {
            try {
                mService.onPostMessage(mSessionBinder, message, extras);
            } catch (RemoteException e) {
                return false;
            }
        }
        return true;
    }
Posts a message to the client. This should be called when a tab controlled by related
{@link CustomTabsSession} has sent a postMessage. If postMessage() is called from a single
thread, then the messages will be posted in the same order.

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