Web Animations: Construct AnimatableValues from RenderStyles

This introduces the API for constructing AnimatableValues out of
RenderStyles. 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/21768003

git-svn-id: svn://svn.chromium.org/blink/trunk@155481 bbb929c8-8fbe-4397-9dbb-9b2b20218538