Move Document code out of Node.cpp

Move Document code out of Node.cpp and into Document.cpp for clarity. There is
no reason (e.g. inlining) requiring that this code be in Node.cpp.

R=adamk@chromium.org, adamk, eseidel

Review URL: https://codereview.chromium.org/178543006

git-svn-id: svn://svn.chromium.org/blink/trunk@168421 bbb929c8-8fbe-4397-9dbb-9b2b20218538