Clean parsing for -webkit-transform-origin*

The indirection of parseWebkitTransformOrigin only adds complexity.

BUG=404023

Review URL: https://codereview.chromium.org/474873002

git-svn-id: svn://svn.chromium.org/blink/trunk@180323 bbb929c8-8fbe-4397-9dbb-9b2b20218538