Web Animations: Add CSSAnimatableValueFactory API (partial implementation)

This introduces the API for constructing AnimatableValues out of
RenderStyles and CSS Values. Basic support for Lengths and
AnimatableNumbers implemented.

Further coverage of all CSS properties and animatable CSS value types
to be accomplished by generated code in future.

This patch is part of replacing CSSPropertyAnimation with
AnimatableValues for Web Animations.
http://dev.w3.org/fxtf/web-animations/

BUG=258896

Review URL: https://chromiumcodereview.appspot.com/19744002

git-svn-id: svn://svn.chromium.org/blink/trunk@155408 bbb929c8-8fbe-4397-9dbb-9b2b20218538