scopingNodeFor should not use null for document scope.

If scopingNodeFor returns null, we should skip the CSSStyleSheet.

BUG=320929,321495

Review URL: https://codereview.chromium.org/76123002

git-svn-id: svn://svn.chromium.org/blink/trunk@162437 bbb929c8-8fbe-4397-9dbb-9b2b20218538