Web Animations: Support player finish events on cancelled animations

Calling cancel, or setting the source to null, causes an immediate finish event.

(If the the currentTime is negative, the finish event does not fire until currentTime reaches 0.)

BUG=356120

Review URL: https://codereview.chromium.org/226453007

git-svn-id: svn://svn.chromium.org/blink/trunk@171225 bbb929c8-8fbe-4397-9dbb-9b2b20218538