[Android] Allow constructor of Notifications on Android.

We support notifications in Android but upstream doesn't so we
should make sure they are enabled.

We should really consider if the "desktop" notifications are the way
forward for us in Android now that we have ServiceWorker and
push notifications.

https://codereview.chromium.org/920153002

BUG=XWALK-3653

content/child/runtime_features.cc

@@ -36,9 +36,6 @@ static void SetRuntimeFeatureDefaultsForPlatform() {
   WebRuntimeFeatures::enableOrientationEvent(true);
   WebRuntimeFeatures::enableFastMobileScrolling(true);
   WebRuntimeFeatures::enableMediaCapture(true);
-  // Android won't be able to reliably support non-persistent notifications, the
-  // intended behavior for which is in flux by itself.
-  WebRuntimeFeatures::enableNotificationConstructor(false);
   WebRuntimeFeatures::enableNewMediaPlaybackUi(true);
   // Android does not yet support switching of audio output devices
   WebRuntimeFeatures::enableAudioOutputDevices(false);