Remove RefCountedGarbageCollected from EventTarget objects

Now that EventTarget is on-heap, we don't need to use RefCountedGarbageCollected for classes that derive EventTargets.

Another reason why these classes have to be RefCountedGarbageCollected is that they derive ActiveDOMObject, and ActiveDOMObject::setPendingActivity calls ref(). However, they don't need to be RefCountedGarbageCollected unless they actually use setPendingActivity(), so this CL just removes RefCountedGarbageCollected from classes that derive ActiveDOMObject but don't use setPendingActivity().

BUG=340522

Review URL: https://codereview.chromium.org/329853004

git-svn-id: svn://svn.chromium.org/blink/trunk@176011 bbb929c8-8fbe-4397-9dbb-9b2b20218538