Remove StyleResolver::ensureScopedStyleResolver

This makes the callers less awkward, and also removes this API from the
public interface of StyleResolver which is out of hand.

Review URL: https://codereview.chromium.org/356853004

git-svn-id: svn://svn.chromium.org/blink/trunk@177037 bbb929c8-8fbe-4397-9dbb-9b2b20218538