Beef up keyframe animation LayoutTest

This change simply documents current behavior. A couple of FIXMEs have been
added for possible changes to improve compatibility with the spec or other
browsers.

R=apavlov

Review URL: https://chromiumcodereview.appspot.com/18864008

git-svn-id: svn://svn.chromium.org/blink/trunk@154109 bbb929c8-8fbe-4397-9dbb-9b2b20218538