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

Source/modules/serviceworkers/InstallEvent.h

Index: Source/modules/serviceworkers/InstallEvent.h
diff --git a/Source/modules/serviceworkers/InstallEvent.h b/Source/modules/serviceworkers/InstallEvent.h
index f282f9c769242ea4c9ceef957a95d64d28b2e424..fddd56d2d74ee8e1efde82d8b3f4ef0b4e694165 100644
--- a/Source/modules/serviceworkers/InstallEvent.h
+++ b/Source/modules/serviceworkers/InstallEvent.h
@@ -51,6 +51,8 @@ public:
 
     virtual const AtomicString& interfaceName() const OVERRIDE;
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     InstallEvent();
     InstallEvent(const AtomicString& type, const EventInit&, PassRefPtr<WaitUntilObserver>);