Move Web Animations methods out of StyleResolver.

Move the methods styleForKeyframe, matchScopedKeyframesRule,
GenerateTimingFunction, and resolveKeyframes out of StyleResolver
and into CSSAnimations. Expose methods and types in StyleResolver
as needed by the transplanted methods.

Change is made in preparation for adding use-counter analytics to
Web Animations, to avoid adding more WA-specific code to the Style
Resolver.

BUG=

Review URL: https://codereview.chromium.org/57593002

git-svn-id: svn://svn.chromium.org/blink/trunk@161706 bbb929c8-8fbe-4397-9dbb-9b2b20218538