ui: Cleanup class/struct forward declarations

ash/mus/bridge/wm_window_mus.h

Index: ash/mus/bridge/wm_window_mus.h
diff --git a/ash/mus/bridge/wm_window_mus.h b/ash/mus/bridge/wm_window_mus.h
index 3331a3e10064b6994c150d1c55f916fef1eda16b..2b2d2251043d7e7a4c86dea2a18f1551615fb4e4 100644
--- a/ash/mus/bridge/wm_window_mus.h
+++ b/ash/mus/bridge/wm_window_mus.h
@@ -152,11 +152,6 @@ class WmWindowMus : public WmWindow, public ui::WindowObserver {
   void SetColorProperty(WmWindowProperty key, SkColor value) override;
   int GetIntProperty(WmWindowProperty key) override;
   void SetIntProperty(WmWindowProperty key, int value) override;
-  std::string GetStringProperty(WmWindowProperty key) override;
-  void SetStringProperty(WmWindowProperty key,
-                         const std::string& value) override;
-  gfx::ImageSkia GetWindowIcon() override;
-  gfx::ImageSkia GetAppIcon() override;
   const wm::WindowState* GetWindowState() const override;
   WmWindow* GetToplevelWindow() override;
   WmWindow* GetToplevelWindowForFocus() override;
@@ -167,7 +162,6 @@ class WmWindowMus : public WmWindow, public ui::WindowObserver {
   const WmWindow* GetParent() const override;
   const WmWindow* GetTransientParent() const override;
   std::vector<WmWindow*> GetTransientChildren() override;
-  bool MoveToEventRoot(const ui::Event& event) override;
   void SetLayoutManager(
       std::unique_ptr<WmLayoutManager> layout_manager) override;
   WmLayoutManager* GetLayoutManager() override;