Extract shell methods from ApplicationImpl into a base class, and pass this to Initialize() instead.

ui/views/mus/surface_context_factory.h

Index: ui/views/mus/surface_context_factory.h
diff --git a/ui/views/mus/surface_context_factory.h b/ui/views/mus/surface_context_factory.h
index c7497b97c860adf4321b5bf1107407f82123215e..72c85fde9c0aa0610a59d06cbef7eabd7c271e26 100644
--- a/ui/views/mus/surface_context_factory.h
+++ b/ui/views/mus/surface_context_factory.h
@@ -16,12 +16,8 @@
 #include "ui/views/mus/surface_binding.h"
 
 namespace mojo {
-namespace shell {
-namespace mojom {
 class Shell;
 }
-}
-}
 
 namespace mus {
 class Window;
@@ -31,7 +27,7 @@ namespace views {
 
 class VIEWS_MUS_EXPORT SurfaceContextFactory : public ui::ContextFactory {
  public:
-  SurfaceContextFactory(mojo::shell::mojom::Shell* shell,
+  SurfaceContextFactory(mojo::Shell* shell,
                         mus::Window* window,
                         mus::mojom::SurfaceType surface_type);
   ~SurfaceContextFactory() override;