Web Animations: define distance for AnimatableColor

Distance is defined in
http://dev.w3.org/fxtf/web-animations/ section 4.1.3.5

Web Animations use property distance calculations in paced keyframe animation effects, in assigning the timing for keyframes so that a 'paced' property changes at constant velocity.

BUG=348896

Review URL: https://codereview.chromium.org/186463002

git-svn-id: svn://svn.chromium.org/blink/trunk@168559 bbb929c8-8fbe-4397-9dbb-9b2b20218538