private final boolean notifyMessageChannelReadyInternal(Bundle extras) {
        if (!isBoundToService()) return false;
        synchronized (mLock) {
            try {
                mService.onMessageChannelReady(mSessionBinder, extras);
            } catch (RemoteException e) {
                return false;
            }
        }
        return true;
    }
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