Web Animations CSS: Assert underlying value is not used

This assert makes it clear we don't expect CompositeAdd keyframes to
be used in the CSS transitions and animations implementation.

BUG=258896

Review URL: https://chromiumcodereview.appspot.com/22707006

git-svn-id: svn://svn.chromium.org/blink/trunk@155855 bbb929c8-8fbe-4397-9dbb-9b2b20218538