Avoid sub-tree style recalc for ::-webkit-resizer.

The presence of the ::-webkit-resizer pseudo element should not trigger a
sub-tree style recalc from style invalidation.

This CL depends on https://codereview.chromium.org/639293008/

R=esprehn@chromium.org,chrishtr@chromium.org
BUG=425557

Review URL: https://codereview.chromium.org/663313004

git-svn-id: svn://svn.chromium.org/blink/trunk@184442 bbb929c8-8fbe-4397-9dbb-9b2b20218538