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

Source/modules/serviceworkers/InstallEvent.cpp

Index: Source/modules/serviceworkers/InstallEvent.cpp
diff --git a/Source/modules/serviceworkers/InstallEvent.cpp b/Source/modules/serviceworkers/InstallEvent.cpp
index e219c45dfd271f0bc64f919dbd8dbfc206eb4140..5a476b84e87dc1b23a0eb458d01846da5917e26e 100644
--- a/Source/modules/serviceworkers/InstallEvent.cpp
+++ b/Source/modules/serviceworkers/InstallEvent.cpp
@@ -83,4 +83,9 @@ InstallEvent::InstallEvent(const AtomicString& type, const EventInit& initialize
     ScriptWrappable::init(this);
 }
 
+void InstallEvent::trace(Visitor* visitor)
+{
+    InstallPhaseEvent::trace(visitor);
+}
+
 } // namespace WebCore