Bug 1333590 - 6. Remove GeckoEventListener and NativeEventListener; r=snorp r=sebastian

Remove GeckoEventListener and NativeEventListener now that we uniformly
use BundleEventListener. Also remove related classes NativeJSContainer,
NativeJSObject, and GeckoRequest, as well as related tests and C++
code.

The "Messaging" object in Messaging.jsm is replaced with a dummy object
that redirect calls to the global and/or window event dispatcher.

widget/android/EventDispatcher.cpp

@@ -753,7 +753,7 @@ EventDispatcher::Dispatch(const nsAString& aEvent, JS::HandleValue aData,
                         aCallback, mDOMWindow));
     }
 
-    mDispatcher->DispatchToThreads(aEvent, /* js */ nullptr, data, callback);
+    mDispatcher->DispatchToThreads(aEvent, data, callback);
     return NS_OK;
 }