Web Animations: Add Player::finished()

Spec at http://dev.w3.org/fxtf/web-animations/#widl-Player-finished.

BUG=334926

Review URL: https://codereview.chromium.org/149933002

git-svn-id: svn://svn.chromium.org/blink/trunk@166175 bbb929c8-8fbe-4397-9dbb-9b2b20218538