Make Event RefCountedGarbageCollected and implement trace() methods to the Event hierarcy

Source/modules/serviceworkers/InstallPhaseEvent.h

Index: Source/modules/serviceworkers/InstallPhaseEvent.h
diff --git a/Source/modules/serviceworkers/InstallPhaseEvent.h b/Source/modules/serviceworkers/InstallPhaseEvent.h
index da6f057a2cb65ff028f4bfd4d5648350850834d9..5b269ad082ee008dbba491121589076a799daa11 100644
--- a/Source/modules/serviceworkers/InstallPhaseEvent.h
+++ b/Source/modules/serviceworkers/InstallPhaseEvent.h
@@ -46,6 +46,8 @@ public:
 
     void waitUntil(const ScriptValue&);
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 protected:
     InstallPhaseEvent();
     InstallPhaseEvent(const AtomicString& type, const EventInit&, PassRefPtr<WaitUntilObserver>);