Simplify compareDocumentPosition

We can just use a convenient default parameter and remove compareDocumentPositionInternal. That latter I dislike since although it mentions Internal it is public.

Review URL: https://codereview.chromium.org/407093010

git-svn-id: svn://svn.chromium.org/blink/trunk@178709 bbb929c8-8fbe-4397-9dbb-9b2b20218538