Revert Event back (from RefCountedWillBeRefCountedGarbageCollected) to RefCounted

Moving Event to oilpan's heap caused a lot of crashes/failures. So this CL reverts Event back to RefCounted.

This CL is a revert for https://codereview.chromium.org/182063004/. However, this CL does not revert trace() methods in the original CL because there is no harm in having trace() methods in the code base. (If I revert trace() methods, I have to resolve a bunch of conflicts manually, which will be more dangerous than leaving trace() methods in the code base.)

BUG=340522
NOTRY=true

Review URL: https://codereview.chromium.org/185463005

git-svn-id: svn://svn.chromium.org/blink/trunk@168354 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/events/Event.h

@@ -44,7 +44,7 @@ struct EventInit {
     bool cancelable;
 };
 
-class Event : public RefCountedWillBeRefCountedGarbageCollected<Event>,  public ScriptWrappable {
+class Event : public RefCounted<Event>,  public ScriptWrappable {
 public:
     enum PhaseType {
         NONE                = 0,

third_party/WebKit/Source/core/events/MouseEvent.h

@@ -91,6 +91,8 @@ public:
     virtual bool isDragEvent() const OVERRIDE FINAL;
     virtual int which() const OVERRIDE FINAL;
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 protected:
     MouseEvent(const AtomicString& type, bool canBubble, bool cancelable, PassRefPtr<AbstractView>,
         int detail, int screenX, int screenY, int pageX, int pageY,
@@ -102,8 +104,6 @@ protected:
 
     MouseEvent();
 
-    virtual void trace(Visitor*) OVERRIDE;
-
 private:
     unsigned short m_button;
     bool m_buttonDown;