Web Animations: Support animation of flood-opacity

Note that the existing implementation fails to clamp to the range 0-1.

BUG=257591

Review URL: https://codereview.chromium.org/26340006

git-svn-id: svn://svn.chromium.org/blink/trunk@159643 bbb929c8-8fbe-4397-9dbb-9b2b20218538