@Override
        public void onPostMessage(String message, Bundle extras) {}
Both {@link #mCallbackBinder} and {@link #mSessionId} are used as session ID.
At least one of the ID should be not null. If {@link #mSessionId} is null,
the session will be invalidated as soon as the client goes away.
Otherwise the browser will attempt to keep the session parameters,
but it might drop them to reclaim resources
customtabs/src/android/support/customtabs/CustomTabsSessionToken.java