mus: Fix handled status in UI event ack, add MessageLoop::NestingObserver

ui/views/mus/platform_window_mus.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_VIEWS_MUS_PLATFORM_WINDOW_MUS_H_
 #define UI_VIEWS_MUS_PLATFORM_WINDOW_MUS_H_
 
 #include <stdint.h>
 
 #include <string>
@@ skipping 46 matching lines @@
   void ToggleFullscreen() override;
   void Maximize() override;
   void Minimize() override;
   void Restore() override;
   void SetCursor(ui::PlatformCursor cursor) override;
   void MoveCursorTo(const gfx::Point& location) override;
   void ConfineCursorToBounds(const gfx::Rect& bounds) override;
   ui::PlatformImeController* GetPlatformImeController() override;
 
  private:
+  friend class PlatformWindowMusTest;
+
   void SetShowState(mus::mojom::ShowState show_state);
 
   // mus::WindowObserver:
   void OnWindowDestroyed(mus::Window* window) override;
   void OnWindowBoundsChanged(mus::Window* window,
                              const gfx::Rect& old_bounds,
                              const gfx::Rect& new_bounds) override;
   void OnWindowFocusChanged(mus::Window* gained_focus,
                             mus::Window* lost_focus) override;
   void OnWindowPredefinedCursorChanged(mus::Window* window,
@@ skipping 25 matching lines @@
 #ifndef NDEBUG
   std::unique_ptr<base::WeakPtrFactory<PlatformWindowMus>> weak_factory_;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(PlatformWindowMus);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_MUS_PLATFORM_WINDOW_MUS_H_