Have Node::document() return a reference instead of a pointer

Have Node::document() return a reference instead of a pointer now that the
method can no longer return NULL. DocumentType Nodes have been recently
updated so that they always have an associated document (as per DOM4).

R=abarth@chromium.org
BUG=281400

Review URL: https://codereview.chromium.org/23819007

git-svn-id: svn://svn.chromium.org/blink/trunk@157019 bbb929c8-8fbe-4397-9dbb-9b2b20218538