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

This reverts commit deb25381d69b6bed38d488b37e2325c3905624c1.

BUG=none
TBR=ch.dumez@sisa.samsung.com

Review URL: https://codereview.chromium.org/23567008

git-svn-id: svn://svn.chromium.org/blink/trunk@157021 bbb929c8-8fbe-4397-9dbb-9b2b20218538