Extract AnimationValue from GraphicsLayer.h to AnimationValue.h

In addtion, extract KeyframeValueList from GraphicsLayer.h to
KeyframeValueList.h

Review URL: https://chromiumcodereview.appspot.com/23460028

git-svn-id: svn://svn.chromium.org/blink/trunk@157607 bbb929c8-8fbe-4397-9dbb-9b2b20218538