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

Source/modules/serviceworkers/InstallPhaseEvent.cpp

Index: Source/modules/serviceworkers/InstallPhaseEvent.cpp
diff --git a/Source/modules/serviceworkers/InstallPhaseEvent.cpp b/Source/modules/serviceworkers/InstallPhaseEvent.cpp
index efdfc92de96167828e2889959233256d5fbbf23e..085170deddb460776e6c478dd554c5dfc53a636e 100644
--- a/Source/modules/serviceworkers/InstallPhaseEvent.cpp
+++ b/Source/modules/serviceworkers/InstallPhaseEvent.cpp
@@ -60,4 +60,9 @@ InstallPhaseEvent::InstallPhaseEvent(const AtomicString& type, const EventInit&
 {
 }
 
+void InstallPhaseEvent::trace(Visitor* visitor)
+{
+    Event::trace(visitor);
+}
+
 } // namespace WebCore