Bug 1492894 - part 4 - eliminate already_AddRefed variables in dom/; r=mccr8

We need to disallow these to fix our static analysis, which should have
already been disallowing them.

dom/canvas/OffscreenCanvas.cpp

@@ -128,7 +128,7 @@ OffscreenCanvas::GetContext(JSContext* aCx,
     return nullptr;
   }
 
-  already_AddRefed<nsISupports> result =
+  RefPtr<nsISupports> result =
     CanvasRenderingContextHelper::GetContext(aCx,
                                              aContextId,
                                              aContextOptions,
@@ -166,7 +166,7 @@ OffscreenCanvas::GetContext(JSContext* aCx,
     }
   }
 
-  return result;
+  return result.forget();
 }
 
 already_AddRefed<nsICanvasRenderingContextInternal>

dom/media/webaudio/PannerNode.cpp

@@ -104,9 +104,9 @@ public:
       return;
     }
     // HRTFDatabaseLoader needs to be fetched on the main thread.
-    already_AddRefed<HRTFDatabaseLoader> loader =
+    RefPtr<HRTFDatabaseLoader> loader =
       HRTFDatabaseLoader::createAndLoadAsynchronouslyIfNecessary(NodeMainThread()->Context()->SampleRate());
-    mHRTFPanner = new HRTFPanner(NodeMainThread()->Context()->SampleRate(), std::move(loader));
+    mHRTFPanner = new HRTFPanner(NodeMainThread()->Context()->SampleRate(), loader.forget());
   }
 
   void SetInt32Parameter(uint32_t aIndex, int32_t aParam) override