Implement DeferredAnimatableValue

This sentinal AnimatableValue marks points in a KeyframeVector that need
to be replaced with a snapshot of the element's current computed
CSSValue once such values have been applied during
applyMatchedProperties().

BUG=232273

Review URL: https://chromiumcodereview.appspot.com/19367002

git-svn-id: svn://svn.chromium.org/blink/trunk@154872 bbb929c8-8fbe-4397-9dbb-9b2b20218538