Declare onFinish event handler for AnimationPlayer.

Finish events occur when a sample occurs that causes a player to be newly limited or newly disassociated with a timeline. 
(From section 3.5.4 Player events 
http://dev.w3.org/fxtf/web-animations/#player-events
and section 5.19 The AnimationPlayerEvent interface
http://dev.w3.org/fxtf/web-animations/#the-animationplayerevent-interface )

Subsequent CL 216203003 dispatches the events declared here.

BUG=356120

Review URL: https://codereview.chromium.org/210363007

git-svn-id: svn://svn.chromium.org/blink/trunk@170518 bbb929c8-8fbe-4397-9dbb-9b2b20218538