Remove null check on refNode in Range::comparePoint

It is not necessary anymore after https://codereview.chromium.org/410413002/

Review URL: https://codereview.chromium.org/412143007

git-svn-id: svn://svn.chromium.org/blink/trunk@178837 bbb929c8-8fbe-4397-9dbb-9b2b20218538