Use document lifecycle for documentBeingDestroyed.

This is a better signal than clearing the RenderView because
it's set at the beginning of Document::detach. This also
allows for some minor cleanup in Document::detach since
we can allow the regular ContainerNode::detach logic
to clear and destroy the RenderView.

There's still plenty of code that null checks the
document's renderer or the RenderView, but that will
need to be cleaned up on a case-by-case basis.

Review URL: https://codereview.chromium.org/254743002

git-svn-id: svn://svn.chromium.org/blink/trunk@172591 bbb929c8-8fbe-4397-9dbb-9b2b20218538