Refactoring of block_rule_body in CSSGrammar.y.
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@162215 bbb929c8-8fbe-4397-9dbb-9b2b20218538

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk