Store [-webkit-]transform-origin as a TransformOrigin.

This makes it possible to use a converter, and makes
style/animation application cleaner.

Review URL: https://codereview.chromium.org/644953003

git-svn-id: svn://svn.chromium.org/blink/trunk@183651 bbb929c8-8fbe-4397-9dbb-9b2b20218538