@Override
        protected void onPostExecute(Boolean success) {
            if (mCallback != null && !isCancelled()) {
                mCallback.onFinished(success);
            }
        }
Cancels the execution. The callback passed into {@link #execute} won't be called, and
the references to it will be released.
customtabs/src/android/support/customtabs/trusted/splashscreens/SplashImageTransferTask.java