@Override
    protected int postMessage(CustomTabsSessionToken sessionToken, String message, Bundle extras) {
        if (!mPostMessageRequested) return CustomTabsService.RESULT_FAILURE_DISALLOWED;
        return CustomTabsService.RESULT_SUCCESS;
    }
Returns the instance of the Service. Returns null if it hasn't been bound yet.
customtabs/tests/src/android/support/customtabs/TestCustomTabsService.java