Web Animations CSS: Support animation of fill-opacity

Note that we clamp values to the range 0-1, the existing implementation
incorrectly produces values outside this range. New behavior matches Gecko.

BUG=257591

Review URL: https://codereview.chromium.org/23542039

git-svn-id: svn://svn.chromium.org/blink/trunk@158601 bbb929c8-8fbe-4397-9dbb-9b2b20218538