Web Animations: Defer cancellation of compositor animation until after compositing update

Removes illegal compositing state queries.

BUG=339847

Review URL: https://codereview.chromium.org/239453006

git-svn-id: svn://svn.chromium.org/blink/trunk@174779 bbb929c8-8fbe-4397-9dbb-9b2b20218538