Add unit test for mus_demo.

The unit test starts mus demo and checks that it successfully draws
compositor frames. Test is not setup to run on bots yet.

There is a problem where the WindowTree name isn't set to the client
name when the WindowTree is created for a WindowManager. Once this is
fixed EnsureClientHasDrawnWindow() should be using "mojo:mus_demo" as
the name it's looking for.

Also use this chance to fix some namespace stuff for MusDemo.

BUG=611473

Review-Url: https://codereview.chromium.org/2138433002
Cr-Commit-Position: refs/heads/master@{#405493}

• services/ui/demo/BUILD.gn[diff]
• services/ui/demo/main.cc[diff]
• services/ui/demo/mus_demo.cc[diff]
• services/ui/demo/mus_demo.h[diff]
• services/ui/demo/mus_demo_unittests.cc[Added - diff]
• services/ui/demo/test_manifest.json[Added - diff]