CSSGrammar.y refactoring.

Preparation for https://codereview.chromium.org/71623002/

BUG=317499

Review URL: https://codereview.chromium.org/63773004

git-svn-id: svn://svn.chromium.org/blink/trunk@162120 bbb929c8-8fbe-4397-9dbb-9b2b20218538