mash: Avoid shelf crashes on display removal.

ash/mus/window_manager.h

Index: ash/mus/window_manager.h
diff --git a/ash/mus/window_manager.h b/ash/mus/window_manager.h
index 67ea7ca4d048a0f56241c4f3e56616ca6825be8d..d10f5acd67ad0bc799100dd54710ac122c2ab5e7 100644
--- a/ash/mus/window_manager.h
+++ b/ash/mus/window_manager.h
@@ -12,15 +12,17 @@
 
 #include "base/macros.h"
 #include "base/observer_list.h"
+#include "mojo/public/cpp/bindings/binding.h"
 #include "services/ui/common/types.h"
 #include "services/ui/public/cpp/window_manager_delegate.h"
 #include "services/ui/public/cpp/window_observer.h"
 #include "services/ui/public/cpp/window_tree_client_delegate.h"
+#include "services/ui/public/interfaces/display.mojom.h"
 #include "services/ui/public/interfaces/window_manager.mojom.h"
 
 namespace display {
 class Display;
-class Screen;
+class ScreenBase;
 }
 
 namespace shell {
@@ -48,7 +50,8 @@ class WmTestHelper;
 // the WindowTreeClient.
 class WindowManager : public ui::WindowManagerDelegate,
                       public ui::WindowObserver,
-                      public ui::WindowTreeClientDelegate {
+                      public ui::WindowTreeClientDelegate,
+                      public ui::mojom::DisplayManagerObserver {
  public:
   explicit WindowManager(shell::Connector* connector);
   ~WindowManager() override;
@@ -121,6 +124,12 @@ class WindowManager : public ui::WindowManagerDelegate,
   ui::mojom::EventResult OnAccelerator(uint32_t id,
                                        const ui::Event& event) override;
 
+  // ui::mojom::DisplayManager:
+  void OnDisplays(mojo::Array<ui::mojom::WsDisplayPtr> ws_displays) override;
+  void OnDisplaysChanged(
+      mojo::Array<ui::mojom::WsDisplayPtr> ws_displays) override;
+  void OnDisplayRemoved(int64_t id) override;
+
   shell::Connector* connector_;
 
   ui::WindowTreeClient* window_tree_client_ = nullptr;
@@ -136,7 +145,9 @@ class WindowManager : public ui::WindowManagerDelegate,
 
   base::ObserverList<WindowManagerObserver> observers_;
 
-  std::unique_ptr<display::Screen> screen_;
+  std::unique_ptr<display::ScreenBase> screen_;
+  mojo::Binding<ui::mojom::DisplayManagerObserver>
+      display_manager_observer_binding_;
 
   std::unique_ptr<WmShellMus> shell_;