Impl mus::mojom::GpuService to enable using Chrome IPC version gpu CmdBuf in mus

components/mus/public/cpp/window_observer.h

Index: components/mus/public/cpp/window_observer.h
diff --git a/components/mus/public/cpp/window_observer.h b/components/mus/public/cpp/window_observer.h
index 27e697c9343bfe68c253688307931660b740fbf3..b819071a5a89d799ee881134f62b3384b9a37fe7 100644
--- a/components/mus/public/cpp/window_observer.h
+++ b/components/mus/public/cpp/window_observer.h
@@ -9,6 +9,7 @@
 
 #include <vector>
 
+#include "components/mus/public/cpp/mus_public_export.h"
 #include "components/mus/public/cpp/window.h"
 
 namespace mus {
@@ -25,7 +26,7 @@ class Window;
 // prior to being called, so for example in the case of OnWindowDestroying(),
 // it's possible the window has already been destroyed on the service side.
 
-class WindowObserver {
+class MUS_PUBLIC_EXPORT WindowObserver {
  public:
   struct TreeChangeParams {
     TreeChangeParams();