mus: Introduce AttachSurface to allow creating Surface prior to OnEmbed

components/mus/public/cpp/window.h

Index: components/mus/public/cpp/window.h
diff --git a/components/mus/public/cpp/window.h b/components/mus/public/cpp/window.h
index fef5d10361a9f871c5bbdda7b04472f371073a59..9da43ce6f31e9c5d07e5cb7dde6f8d78401be0c0 100644
--- a/components/mus/public/cpp/window.h
+++ b/components/mus/public/cpp/window.h
@@ -29,6 +29,7 @@ namespace mus {
 class ServiceProviderImpl;
 class WindowObserver;
 class WindowSurface;
+class WindowSurfaceBinding;
 class WindowTreeClientImpl;
 class WindowTreeConnection;
 
@@ -87,6 +88,9 @@ class Window {
 
   scoped_ptr<WindowSurface> RequestSurface(mojom::SurfaceType type);
 
+  void AttachSurface(mojom::SurfaceType type,
+                     scoped_ptr<WindowSurfaceBinding> surface_binding);
+
   // The template-ized versions of the following methods rely on the presence
   // of a mojo::TypeConverter<const std::vector<uint8_t>, T>.
   // Sets a shared property on the window which is sent to the window server and