Refactoring of block_rule_body in CSSGrammar.y.

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

BUG=317499

Review URL: https://codereview.chromium.org/63903004

git-svn-id: svn://svn.chromium.org/blink/trunk@162215 bbb929c8-8fbe-4397-9dbb-9b2b20218538