Oilpan: add missing leftmost trace()s for GC mixins.

Source/modules/mediastream/UserMediaController.h

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *     notice, this list of conditions and the following disclaimer in the
@@ skipping 22 matching lines @@
 
 class MediaDevicesRequest;
 class UserMediaRequest;
 
 class UserMediaController final : public NoBaseWillBeGarbageCollected<UserMediaController>, public WillBeHeapSupplement<LocalFrame> {
     WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(UserMediaController);
     DECLARE_EMPTY_VIRTUAL_DESTRUCTOR_WILL_BE_REMOVED(UserMediaController);
 public:
     static PassOwnPtrWillBeRawPtr<UserMediaController> create(UserMediaClient*);
 
+    virtual void trace(Visitor*) override;
+
     UserMediaClient* client() const { return m_client; }
 
     void requestUserMedia(UserMediaRequest*);
     void cancelUserMediaRequest(UserMediaRequest*);
 
     void requestMediaDevices(MediaDevicesRequest*);
     void cancelMediaDevicesRequest(MediaDevicesRequest*);
 
     void requestSources(MediaStreamTrackSourcesRequest*);
 
@@ skipping 27 matching lines @@
 }
 
 inline void UserMediaController::requestSources(MediaStreamTrackSourcesRequest* request)
 {
     m_client->requestSources(request);
 }
 
 } // namespace blink
 
 #endif // UserMediaController_h