Bug 1337910 - Notify observers from Gecko event queue; r=snorp

Call NotifyObservers from the Gecko event queue instead of the Android
event queue, to better support Promises in notified JS code.

mobile/android/geckoview/src/main/java/org/mozilla/gecko/GeckoAppShell.java

@@ -254,7 +254,7 @@ public class GeckoAppShell
     @WrapForJNI(calledFrom = "gecko")
     public static native void syncNotifyObservers(String topic, String data);
 
-    @WrapForJNI(stubName = "NotifyObservers", dispatchTo = "gecko")
+    @WrapForJNI(stubName = "NotifyObservers", dispatchTo = "proxy")
     private static native void nativeNotifyObservers(String topic, String data);
 
     @RobocopTarget

widget/android/GeneratedJNIWrappers.h

@@ -1520,7 +1520,7 @@ public:
         static const mozilla::jni::CallingThread callingThread =
                 mozilla::jni::CallingThread::ANY;
         static const mozilla::jni::DispatchTarget dispatchTarget =
-                mozilla::jni::DispatchTarget::GECKO;
+                mozilla::jni::DispatchTarget::PROXY;
     };
 
     struct NotifyAlertListener_t {

widget/android/nsAppShell.cpp

@@ -259,6 +259,13 @@ public:
         NotifyObservers(aTopic, aData);
     }
 
+    template<typename Functor>
+    static void OnNativeCall(Functor&& aCall)
+    {
+        MOZ_ASSERT(aCall.IsTarget(&NotifyObservers));
+        NS_DispatchToMainThread(NS_NewRunnableFunction(mozilla::Move(aCall)));
+    }
+
     static void NotifyObservers(jni::String::Param aTopic,
                                 jni::String::Param aData)
     {