Have Node::diff() take a Document reference in argument

Have Node::diff() take a Document reference in argument instead of a pointer
as it can never be NULL. This avoid avoids having to convert a reference
into a pointer at call sites.

R=tkent
BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23549008

git-svn-id: svn://svn.chromium.org/blink/trunk@157077 bbb929c8-8fbe-4397-9dbb-9b2b20218538