CSSGrammary.y: group
error recovery for @-rules to reduce the recovery code spreading

Error in parsing the example provided in the issue happens because error recovery in the rules block_rule_body and region_block_rule_body is designed for error recovery in ruleset (if rule starts with a selector or error). But this doesn't work for @-rules, as they have their own error recovery grammar.

To fix this, @-rules error recovery should be split into header recovery and full rule recovery parts, and then the header recovery part should be reused in the rules block_rule_body and region_block_rule_body.

This change moves the error recovery only for rules from region_block_valid_rule, to keep the grammar equivalent (non-equivalent changes will be made separately).

BUG=321540

Review URL: https://codereview.chromium.org/78123002

git-svn-id: svn://svn.chromium.org/blink/trunk@162379 bbb929c8-8fbe-4397-9dbb-9b2b20218538