The underlying problem is that we lacked a RenderObjectWrapper for the
RenderView, which meant we couldn't handle changing the RenderObject that was
the root of the RenderView. This CL introduces a RenderViewWrapper and uses it
in a new AppContainer widget root. This change allows us to make App a
non-magical Component that is inserted into the AppContainer in the newly
introduced runApp function.

R=ianh@google.com

Review URL: https://codereview.chromium.org/1184823006.