Node::didMoveToNewDocument should take a reference since oldDocument can never be null

Element::didMoveToNewDocument was already assuming oldDocument was non-null,
so this should be fairly safe.

There are still some ampersands in Node::didMoveToNewDocument mostly relating
to WheelController. Seems like we should give some though to how Supplements
work with references.

R=esprehn

Review URL: https://codereview.chromium.org/27252002

git-svn-id: svn://svn.chromium.org/blink/trunk@159625 bbb929c8-8fbe-4397-9dbb-9b2b20218538