Web Animations: Add Player::finish()

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

BUG=334926

Review URL: https://codereview.chromium.org/151203003

git-svn-id: svn://svn.chromium.org/blink/trunk@166396 bbb929c8-8fbe-4397-9dbb-9b2b20218538