Reland of "Mustash: Ensure surfaces submitted to Mus by WM and embedders contain Surfaces with embe…

ui/aura/mus/window_port_mus.h

Index: ui/aura/mus/window_port_mus.h
diff --git a/ui/aura/mus/window_port_mus.h b/ui/aura/mus/window_port_mus.h
index fd598d0f5a2bc09238754b8555e13b94fa66e0b8..f687f42b04f99e1b550a7262efcdad7edb7369d2 100644
--- a/ui/aura/mus/window_port_mus.h
+++ b/ui/aura/mus/window_port_mus.h
@@ -26,6 +26,7 @@
 
 namespace aura {
 
+class ClientSurfaceEmbedder;
 class PropertyConverter;
 class SurfaceIdHandler;
 class Window;
@@ -243,6 +244,9 @@ class AURA_EXPORT WindowPortMus : public WindowPort, public WindowMus {
 
   Window* window_ = nullptr;
 
+  // Used when this window is embedding a client.
+  std::unique_ptr<ClientSurfaceEmbedder> client_surface_embedder;
+
   ServerChangeIdType next_server_change_id_ = 0;
   ServerChanges server_changes_;