Changing some more [Pass]RefPtr's to transition types for the CSSValue's subclasses.

Also had to add a few additional helper methods in CSSValue, Visitor and RawPtr.

R=ager@chromium.org, erik.corry@gmail.com, haraken@chromium.org, jochen@chromium.org, vegorov@chromium.org, zerny@chromium.org
BUG=341815

Review URL: https://codereview.chromium.org/157563002

git-svn-id: svn://svn.chromium.org/blink/trunk@166829 bbb929c8-8fbe-4397-9dbb-9b2b20218538