Make getDestinationInsertionPoints() skip insertion points in user-agent shadow trees rather than returning an empty list.

BUG=271253

Review URL: https://chromiumcodereview.appspot.com/22481010

git-svn-id: svn://svn.chromium.org/blink/trunk@155988 bbb929c8-8fbe-4397-9dbb-9b2b20218538