Web Animations CSS: Support animation of flood-color, lighting-color and stop-color

Also add tests for text-decoration-color and column-rule-color.

Note that animation of text-decoration-color is currently broken. See
http://crbug.com/305909. It is also currently behind the
--enable-experimental-web-platform-features runtime flag.

BUG=257591
R=dstockwell@chromium.org, timloh@chromium.org, dstockwell, timloh

Review URL: https://codereview.chromium.org/26790002

git-svn-id: svn://svn.chromium.org/blink/trunk@159426 bbb929c8-8fbe-4397-9dbb-9b2b20218538