Web Animations: Rename TimedItem to AnimationSource

Per http://lists.w3.org/Archives/Public/public-fx/2014AprJun/0088.html

Review URL: https://codereview.chromium.org/293893003

git-svn-id: svn://svn.chromium.org/blink/trunk@174534 bbb929c8-8fbe-4397-9dbb-9b2b20218538