Oilpan: reliably track listening PlatformEventControllers.

Source/core/frame/PlatformEventController.h

Index: Source/core/frame/PlatformEventController.h
diff --git a/Source/core/frame/PlatformEventController.h b/Source/core/frame/PlatformEventController.h
index 959a0a2650e9cc9567b9f869b7cf9ecbfd4dbe99..4beec197ed694c590f6be1f13a246559ab8ffa2f 100644
--- a/Source/core/frame/PlatformEventController.h
+++ b/Source/core/frame/PlatformEventController.h
@@ -15,7 +15,7 @@ namespace blink {
 // 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();
@@ -23,6 +23,8 @@ public:
     // This is called when new data becomes available.
     virtual void didUpdateData() = 0;
 
+    virtual void trace(Visitor*) { }
+
 protected:
     explicit PlatformEventController(Page*);
     virtual ~PlatformEventController();