Revert 159188 "Document should not use confusingAndOftenMisusedA..."

This change is likely the cause of crbug.com/306566 (our top
crasher). Unfortunately nobody can repro this bug so this is
an optimistic revert.

> 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

TBR=esprehn@chromium.org

Review URL: https://codereview.chromium.org/27509004

git-svn-id: svn://svn.chromium.org/blink/trunk@159803 bbb929c8-8fbe-4397-9dbb-9b2b20218538