Handle modified displays in mustash.

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 f5cde47afbb9781e835a06a7a69fbbec3d29142c..1f94114c66e653933109c7018948b803f42c838d 100644
--- a/ash/mus/window_manager.h
+++ b/ash/mus/window_manager.h
@@ -16,6 +16,7 @@
 #include "services/ui/common/types.h"
 #include "services/ui/public/cpp/window_manager_delegate.h"
 #include "services/ui/public/cpp/window_tree_client_delegate.h"
+#include "services/ui/public/interfaces/display/display_controller.mojom.h"
 #include "services/ui/public/interfaces/window_manager.mojom.h"
 
 namespace base {
@@ -91,6 +92,10 @@ class WindowManager : public ui::WindowManagerDelegate,
   void AddObserver(WindowManagerObserver* observer);
   void RemoveObserver(WindowManagerObserver* observer);
 
+  // Returns the DisplayController interface if available. Will be null if no
+  // service_manager::Connector was available, for example in some tests.
+  display::mojom::DisplayController* GetDisplayController();
+
  private:
   friend class WmTestHelper;
 
@@ -141,6 +146,7 @@ class WindowManager : public ui::WindowManagerDelegate,
   void OnWmNewDisplay(ui::Window* window,
                       const display::Display& display) override;
   void OnWmDisplayRemoved(ui::Window* window) override;
+  void OnWmDisplayModified(const display::Display& display) override;
   void OnWmPerformMoveLoop(ui::Window* window,
                            ui::mojom::MoveLoopSource source,
                            const gfx::Point& cursor_location,
@@ -150,6 +156,7 @@ class WindowManager : public ui::WindowManagerDelegate,
                                        const ui::Event& event) override;
 
   service_manager::Connector* connector_;
+  display::mojom::DisplayControllerPtr display_controller_;
 
   std::unique_ptr<ui::WindowTreeClient> window_tree_client_;