Move ShowState-related code from PlatformWindowMus to NativeWidgetMus

ui/views/mus/window_tree_host_mus.h

Index: ui/views/mus/window_tree_host_mus.h
diff --git a/ui/views/mus/window_tree_host_mus.h b/ui/views/mus/window_tree_host_mus.h
index bc678752ef238f14f67c5122ad7b811cf31fb4fd..30da7ed2d3fb0b6b0d99855d1ba1fb6128d281c9 100644
--- a/ui/views/mus/window_tree_host_mus.h
+++ b/ui/views/mus/window_tree_host_mus.h
@@ -33,19 +33,16 @@ class VIEWS_MUS_EXPORT WindowTreeHostMus : public aura::WindowTreeHostPlatform {
   ~WindowTreeHostMus() override;
 
   PlatformWindowMus* platform_window();
-  ui::PlatformWindowState show_state() const { return show_state_; }
 
  private:
   // aura::WindowTreeHostPlatform:
   void DispatchEvent(ui::Event* event) override;
   void OnClosed() override;
-  void OnWindowStateChanged(ui::PlatformWindowState new_state) override;
   void OnActivationChanged(bool active) override;
   void OnCloseRequest() override;
 
   NativeWidgetMus* native_widget_;
   std::unique_ptr<InputMethodMUS> input_method_;
-  ui::PlatformWindowState show_state_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowTreeHostMus);
 };