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

- This CL makes Event RefCountedGarbageCollected. In a next CL, I'll change this to GarbageCollectedFinalized.

- This CL just implements trace() methods and doesn't change any RefPtr<XXXEvent> to transition types. I'll do that in a next CL.

- This CL changes Persistent handles in the Event hierarchy to Members.

- The only raw pointer in the Event hierarchy is Event::m_currentTarget. This raw pointer is no problem because it's cleared after the Event is dispatched.

BUG=340522

Review URL: https://codereview.chromium.org/181153003

git-svn-id: svn://svn.chromium.org/blink/trunk@167905 bbb929c8-8fbe-4397-9dbb-9b2b20218538