Web Animations: Make AnimationPlayer an ActiveDOMObject

Previously the AnimationPlayer's finish event could have been dispatched
after the event handlers had been collected. As these events are queued
then dispatched asynchronously via ScriptedAnimationController some
special handling is required (tracking when the event is actually
dispatched).

BUG=376349

Review URL: https://codereview.chromium.org/284323004

git-svn-id: svn://svn.chromium.org/blink/trunk@174943 bbb929c8-8fbe-4397-9dbb-9b2b20218538