Let ScopedStyleResolver::treeScope() return a reference instead of a pointer.

This is a simple refactoring and is a follow-up of r157111.

BUG=none
R=hayato@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23471006

git-svn-id: svn://svn.chromium.org/blink/trunk@157275 bbb929c8-8fbe-4397-9dbb-9b2b20218538