Clean up Document::renderView()

Instead of shadowing the implementations of Document::renderer() and
setRenderer() we manually manage the m_renderView pointer. This is
cleaner and less confusing since setRenderer(0) doesn't do something
special inside Document compared to normal Nodes.

This is in preparation to making Document::renderer() private and
forcing everyone to use ::renderView().

This is part of the reverted r159188 that was not an issue.

Review URL: https://codereview.chromium.org/27474004

git-svn-id: svn://svn.chromium.org/blink/trunk@159894 bbb929c8-8fbe-4397-9dbb-9b2b20218538