Hitting the clip rect cache should be 10% faster

We spend 10% of the time when hitting the ClipRect cache in ref() and deref().
There's no reason to call those functions in the cache hit codepath.

Review URL: https://codereview.chromium.org/349763003

git-svn-id: svn://svn.chromium.org/blink/trunk@176799 bbb929c8-8fbe-4397-9dbb-9b2b20218538