Web Animations CSS: Snapshot missing start/end Keyframes and remove duplicates

Construction of start/end keyframes and normalization is required outside
KeyframeAnimationEffect since CSS Animations have constraints outside the 
Web Animations model.

BUG=258896

Review URL: https://chromiumcodereview.appspot.com/21779003

git-svn-id: svn://svn.chromium.org/blink/trunk@155677 bbb929c8-8fbe-4397-9dbb-9b2b20218538