Web Animations: Avoid leak in AnimationPlayer

AnimationPlayer should stop indicating activity when active dom object is
stopped.

BUG=379032

Review URL: https://codereview.chromium.org/301363002

git-svn-id: svn://svn.chromium.org/blink/trunk@175143 bbb929c8-8fbe-4397-9dbb-9b2b20218538