[Oilpan] Fix wrong usage of HeapCountedSet<EventTarget*>

third_party/WebKit/Source/core/frame/EventHandlerRegistry.h

Index: third_party/WebKit/Source/core/frame/EventHandlerRegistry.h
diff --git a/third_party/WebKit/Source/core/frame/EventHandlerRegistry.h b/third_party/WebKit/Source/core/frame/EventHandlerRegistry.h
index b89ea927a4ad5d8ce3a45e8d2d4b52ba1ac6686d..c517b617322d8c24b0a331b417743455c3d10100 100644
--- a/third_party/WebKit/Source/core/frame/EventHandlerRegistry.h
+++ b/third_party/WebKit/Source/core/frame/EventHandlerRegistry.h
@@ -14,7 +14,10 @@ namespace blink {
 class Document;
 class EventTarget;
 
-typedef HashCountedSet<EventTarget*> EventTargetSet;
+// EventHandlerRegistry::m_targets is handled manually in clearWeakMembers(),
+// so its type can be HeapHashCountedSet<Member<>>, but we use WeakMember<> in this
+// alias for future other use cases.
+using EventTargetSet = WillBeHeapHashCountedSet<RawPtrWillBeWeakMember<EventTarget>>;
 
 // Registry for keeping track of event handlers. Note that only handlers on
 // documents that can be rendered or can receive input (i.e., are attached to a