Web animations: Adds new getAnimatableValue method to Keyframe and removes CompositorAnimations dependency on AnimatableValueKeyframe by using this method instead.

BUG=360471

Review URL: https://codereview.chromium.org/228043002

git-svn-id: svn://svn.chromium.org/blink/trunk@171024 bbb929c8-8fbe-4397-9dbb-9b2b20218538