Make DesktopFactoryOzone instances to respect --ozone-platform
In order to support "use_ozone=1 ichromeos=0" builds,
DesktopFactoryOzone class needs to respect the Ozone platform
backend that is going to run.
For instance, ui::OzonePlatform class (see ui/ozone/) accomplishes
this using the logic provided by ui::OzoneObject.
In short, OzoneObject allows OzonePlatform instances (e.g. wayland,
GDM, x11) to get selected at runtime, obeying either --ozone-platform
command line or using a default value to select the platform backend.
This patcj extends this logic to DesktopFactoryOzone class, as a first
step towards supporting "use_ozone=1 chromeos=0" builds.
DesktopFactoryOzoneWayland is added (with dummy implementations) as
a sample implementation and starting point.