@Override
    protected boolean requestPostMessageChannel(
            CustomTabsSessionToken sessionToken, Uri postMessageOrigin) {
        if (mSession == null) return false;
        mPostMessageRequested = true;
        mSession.getCallback().extraCallback(CALLBACK_BIND_TO_POST_MESSAGE, null);
        return true;
    }
Returns the instance of the Service. Returns null if it hasn't been bound yet.
customtabs/tests/src/android/support/customtabs/TestCustomTabsService.java