Refactors DimWindow and moves to ash/common

ash/mus/bridge/wm_shell_mus.h

Index: ash/mus/bridge/wm_shell_mus.h
diff --git a/ash/mus/bridge/wm_shell_mus.h b/ash/mus/bridge/wm_shell_mus.h
index de6ef61cd636fd0809b6b350e82b074e08e38822..e95209db324d452770c2c26a0440b289eb42154b 100644
--- a/ash/mus/bridge/wm_shell_mus.h
+++ b/ash/mus/bridge/wm_shell_mus.h
@@ -62,7 +62,8 @@ class WmShellMus : public WmShell, public ui::WindowTreeClientObserver {
   }
 
   // WmShell:
-  WmWindow* NewContainerWindow() override;
+  WmWindow* NewWindow(ui::wm::WindowType window_type,
+                      ui::LayerType layer_type) override;
   WmWindow* GetFocusedWindow() override;
   WmWindow* GetActiveWindow() override;
   WmWindow* GetCaptureWindow() override;