Revert 156914 "Web Animations CSS: Implement AnimatableColor"

virtual/web-animations-css/animations/cross-fade-border-image-source.html and
virtual/web-animations-css/animations/interpolation/clip-interpolation.html
had assertion failures.

STDERR: ASSERTION FAILED: value && value->isColor()
STDERR: ../../third_party/WebKit/Source/core/animation/AnimatableColor.h(75) : const WebCore::AnimatableColor* WebCore::toAnimatableColor(const WebCore::AnimatableValue*)

http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&showExpectations=true&showLargeExpectations=true&tests=virtual%2Fweb-animations-css%2Fanimations%2Fcross-fade-border-image-source.html%2Cvirtual%2Fweb-animations-css%2Fanimations%2Finterpolation%2Fclip-interpolation.html

> Web Animations CSS: Implement AnimatableColor
> 
> Add support for animating colors in the new web animations engine.
> We need to friend RenderStyle to support animating the visited and
> unvisited colors separately. Note that we need to update both of them
> at least in the case of displaying visited colors and returning
> up-to-date unvisited colors to getComputedStyle.
> 
> BUG=257591
> 
> Review URL: https://chromiumcodereview.appspot.com/23135015

TBR=timloh@chromium.org

Review URL: https://codereview.chromium.org/23483018

git-svn-id: svn://svn.chromium.org/blink/trunk@156919 bbb929c8-8fbe-4397-9dbb-9b2b20218538