Web Animations API: Add TimedItem bindings for Playback state and calculated timing

Spec: http://dev.w3.org/fxtf/web-animations/#the-timeditem-interface

BUG=334927

Review URL: https://codereview.chromium.org/162483003

git-svn-id: svn://svn.chromium.org/blink/trunk@167143 bbb929c8-8fbe-4397-9dbb-9b2b20218538