Split CSSTokenizer out of CSSParser

This is a first take at moving CSSTokenizer out of CSSParser,
some related functions were not moved along as the patch was
taking epic proportions already.

BUG=320488

Review URL: https://codereview.chromium.org/74943002

git-svn-id: svn://svn.chromium.org/blink/trunk@162252 bbb929c8-8fbe-4397-9dbb-9b2b20218538