Return a reference instead of a pointer in Node::treeScope().

BUG=none
R=hayato@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23878003

git-svn-id: svn://svn.chromium.org/blink/trunk@157111 bbb929c8-8fbe-4397-9dbb-9b2b20218538