Antonio Gomes authored Mar 22, 2017 and Santosh Mahto committed Nov 22, 2017

This CL is the first step towards having 'external window mode' support
in Mus. The existing MusDemo is adapted to test the functionality,
and its respective unit test (mus_demo_unittests) is changed to
exercise  multiple external windows creation in a follow up CL [2].

[2] https://codereview.chromium.org/2715533005/

New code flow:

* MusDemoExternal creates and holds a WindowTreeClient instance.

Note: In Chrome, it also happens in
c/b/ui/views/chrome_browser_main_extra_parts_views.cc,
::ServiceManagerConnectionStarted, when MusClient is created.

* MusDemoExternal calls WindowTreeClient::ConnectViaWindowTreeHostFactory.
This "enters" WindowTreeClient in 'external window mode', and
creates a WindowTreeHostFactoryRegistrar instance.
Through this object, WindowTreeHostFactoryRegistrar::Register is
called, creating WindowTree and WindowTreeHostFactory instances on
the server side, and acquiring WindowTree and
WindowTreeHostFactory mojo handles on the client si...