@Override
            public void extraCallback(final String callbackName, final Bundle args)
                    throws RemoteException {
                if (callback == null) return;
                mHandler.post(new Runnable() {
                    @Override
                    public void run() {
                        callback.extraCallback(callbackName, args);
                    }
                });
            }
Creates a new pending session with an optional callback. This session can be converted to
a standard session using {@link #attachSession} after connection.

{@see PendingSession}
customtabs/src/android/support/customtabs/CustomTabsClient.java