Clear StyleResolverState after each resolve.

This makes sure RenderStyles aren't kept alive unnecessarily by the
two RefPtr<RenderStyles> inside there.  However, it's not enough to
make sure that *all* RenderStyles are destroyed eagerly.

BUG=172011
R=eseidel@chromium.org, esprehn@chromium.org

Review URL: https://codereview.chromium.org/5216392399814656

git-svn-id: svn://svn.chromium.org/blink/trunk@154162 bbb929c8-8fbe-4397-9dbb-9b2b20218538