chromeos: Changes DisplayManagerObserver to have a single function

ui/views/mus/screen_mus.h

Index: ui/views/mus/screen_mus.h
diff --git a/ui/views/mus/screen_mus.h b/ui/views/mus/screen_mus.h
index a5c1978a758e21a860282ccac8e15176aaf16d24..a1eae31511df730ace40968f78b56268a059e02b 100644
--- a/ui/views/mus/screen_mus.h
+++ b/ui/views/mus/screen_mus.h
@@ -29,6 +29,8 @@ class VIEWS_MUS_EXPORT ScreenMus
   void Init(service_manager::Connector* connector);
 
  private:
+  friend class ScreenMusTestApi;
+
   // display::Screen:
   display::Display GetDisplayNearestWindow(
       gfx::NativeWindow window) const override;
@@ -37,13 +39,9 @@ class VIEWS_MUS_EXPORT ScreenMus
   aura::Window* GetWindowAtScreenPoint(const gfx::Point& point) override;
 
   // ui::mojom::DisplayManager:
-  void OnDisplays(std::vector<ui::mojom::WsDisplayPtr> ws_displays,
-                  int64_t primary_display_id,
-                  int64_t internal_display_id) override;
-  void OnDisplaysChanged(
-      std::vector<ui::mojom::WsDisplayPtr> ws_displays) override;
-  void OnDisplayRemoved(int64_t display_id) override;
-  void OnPrimaryDisplayChanged(int64_t primary_display_id) override;
+  void OnDisplaysChanged(std::vector<ui::mojom::WsDisplayPtr> ws_displays,
+                         int64_t primary_display_id,
+                         int64_t internal_display_id) override;
 
   ScreenMusDelegate* delegate_;
   ui::mojom::DisplayManagerPtr display_manager_;