Web Animations CSS: Support animation of stop-opacity

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

BUG=257591

Review URL: https://codereview.chromium.org/25729006

git-svn-id: svn://svn.chromium.org/blink/trunk@158882 bbb929c8-8fbe-4397-9dbb-9b2b20218538