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

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
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=157019

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=157023

[abarth-chromium, 2013-08-30 19:49:02]

I sampled a random subset of these changes, and LGTM

[do-not-use, 2013-08-30 20:52:44]

Committed patchset #4 manually as r157019 (presubmit successful).

[do-not-use, 2013-08-30 22:34:51]

Committed patchset #6 manually as r157023 (presubmit successful).