Missing return after class invalidation short-cut.

scheduleStyleInvalidationForClassChange skipped comparison when oldClasses
was empty, but forgot to return afterwards, causing invalidation sets to be
scheduled twice.

This fix increases runs/s by 10%+ on the added performance test.

R=esprehn@chromium.org,chrishtr@chromium.org
BUG=380874

Review URL: https://codereview.chromium.org/313963006

git-svn-id: svn://svn.chromium.org/blink/trunk@175568 bbb929c8-8fbe-4397-9dbb-9b2b20218538