Align -webkit-transform-origin behaviour with transform-origin behaviour

Currently -webkit-transform-origin is a shorthand while transform-origin
is not. This patch makes the vendor-prefixed property behave the same as
non-prefixed property. This has minor changes to serialization, which
the test changes document.

In doing so we fix a crash in parsing some invalid values.

BUG=405323

Review URL: https://codereview.chromium.org/649153002

git-svn-id: svn://svn.chromium.org/blink/trunk@183588 bbb929c8-8fbe-4397-9dbb-9b2b20218538