Web Animations: Use an IDL enum for AnimationPlayState

BUG=396366

Review URL: https://codereview.chromium.org/562493002

git-svn-id: svn://svn.chromium.org/blink/trunk@181698 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Early out from sheriff-o-matic loop.

Don't compute all the keys for a loop that we almost
always early out from. Also, the early out was wrong because
it was just returning from the anonymous function.

NOTRY=true

Review URL: https://codereview.chromium.org/557173002

git-svn-id: svn://svn.chromium.org/blink/trunk@181699 bbb929c8-8fbe-4397-9dbb-9b2b20218538