views/mus: Install a mus-friendly ContextFactory for tests.
A test running with aura-mus (in aura, views, or ash) does not need a
ui::ContextFactoryPrivate instance. It only needs a ui::ContextFactory.
ui::InProcessContextFactory is not the right implementation to use in a
mus client though, since it directly deals with cc::SurfaceManager,
cc::Display etc., which are not suitable for a mus client. So provide a
simpler implementation of ContextFactory instead in compositor which can
be used from aura, views, and ash.
Use this new ContextFactory for views_mus_unittests and mash_unittests.
This fixes using ui::DrawWaiterForTest in these tests.