Move Event to oilpan's heap

Now there is no test failing when I change RefCounted<Event> to RefCountedGarbageCollected<Event>. Thus this CL does the change.

BUG=341032

Review URL: https://codereview.chromium.org/208173004

git-svn-id: svn://svn.chromium.org/blink/trunk@169941 bbb929c8-8fbe-4397-9dbb-9b2b20218538