Adds a counter for @-webkit-region rule.

@region rule replaced by ::region() functional pseudo-element in http://www.w3.org/TR/css3-regions/

BUG=

Review URL: https://codereview.chromium.org/78773002

git-svn-id: svn://svn.chromium.org/blink/trunk@162406 bbb929c8-8fbe-4397-9dbb-9b2b20218538