mash: Migrate ScopedTargetRootWindow 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 4185b61d30dc09e841402beffbe5bbf881f695f1..4f0bb001e5b8e8eede8f0b1a56efeedb8b5a1224 100644
--- a/ash/mus/bridge/wm_shell_mus.h
+++ b/ash/mus/bridge/wm_shell_mus.h
@@ -61,6 +61,8 @@ class WmShellMus : public WmShell, public ui::WindowTreeClientObserver {
   WmWindow* GetPrimaryRootWindow() override;
   WmWindow* GetRootWindowForDisplayId(int64_t display_id) override;
   WmWindow* GetRootWindowForNewWindows() override;
+  std::unique_ptr<ScopedRootWindowForNewWindows>
+  CreateScopedRootWindowForNewWindows(WmWindow* root) override;
   const DisplayInfo& GetDisplayInfo(int64_t display_id) const override;
   bool IsActiveDisplayId(int64_t display_id) const override;
   bool IsForceMaximizeOnFirstRun() override;