Web Animations: Use counter for AnimationPlayer finish events

The new use counter AnimationPlayerFinishEvent counts pages that use AnimationPlayer finish events.

BUG=356120

Review URL: https://codereview.chromium.org/227163012

git-svn-id: svn://svn.chromium.org/blink/trunk@171135 bbb929c8-8fbe-4397-9dbb-9b2b20218538