Remove CSSTokenizer friendship from CSSParser

This change moves m_internal (added a FIXME about this
horrible boolean) and tightens the API between the 2 classes
as it prevents CSSTokenizer from poking into CSSParser.

BUG=320488

Review URL: https://codereview.chromium.org/77383002

git-svn-id: svn://svn.chromium.org/blink/trunk@162452 bbb929c8-8fbe-4397-9dbb-9b2b20218538