Document should not use confusingAndOftenMisusedAttached()

Switch all uses in Document.cpp of confusingAndOftenMisusedAttached()
to use the new DocumentLifecycle concept. This also switched away from
nulling out the RenderView on Document to indicate "destruction mode",
and now uses the Stopping state of Document which is much nicer and
means the render tree still has access to the root of the tree even
during teardown.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/26347002

git-svn-id: svn://svn.chromium.org/blink/trunk@159188 bbb929c8-8fbe-4397-9dbb-9b2b20218538