Web Animations CSS: Support animation of fill and stroke

Note that we step paints other than rbg colors at 50%. The existing implementation
did not attempt to set these values at all.

BUG=257591

Review URL: https://codereview.chromium.org/25456002

git-svn-id: svn://svn.chromium.org/blink/trunk@158904 bbb929c8-8fbe-4397-9dbb-9b2b20218538