Oilpan: reliably track listening PlatformEventControllers.

Source/core/frame/PlatformEventController.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef PlatformEventController_h
 #define PlatformEventController_h
 
 #include "core/page/PageLifecycleObserver.h"
 #include "platform/Timer.h"
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 // Base controller class for registering controllers with a dispatcher.
 // It watches page visibility and calls stopUpdating when page is not visible.
 // It provides a didUpdateData() callback method which is called when new data
 // it available.
-class PlatformEventController : public PageLifecycleObserver {
+class PlatformEventController : public WillBeGarbageCollectedMixin, public PageLifecycleObserver {
 public:
     void startUpdating();
     void stopUpdating();
 
     // This is called when new data becomes available.
     virtual void didUpdateData() = 0;
 
+    virtual void trace(Visitor*) { }
+
 protected:
     explicit PlatformEventController(Page*);
     virtual ~PlatformEventController();
 
     virtual void registerWithDispatcher() = 0;
     virtual void unregisterWithDispatcher() = 0;
 
     // When true initiates a one-shot didUpdateData() when startUpdating() is called.
     virtual bool hasLastData() = 0;
 
     bool m_hasEventListener;
 
 private:
     // Inherited from PageLifecycleObserver.
     virtual void pageVisibilityChanged() OVERRIDE;
 
     void oneShotCallback(Timer<PlatformEventController>*);
 
     bool m_isActive;
     Timer<PlatformEventController> m_timer;
 };
 
 } // namespace blink
 
 #endif // PlatformEventController_h