mash: Migrate ScopedTargetRootWindow to //ash/common

ash/mus/bridge/wm_shell_mus.cc

Index: ash/mus/bridge/wm_shell_mus.cc
diff --git a/ash/mus/bridge/wm_shell_mus.cc b/ash/mus/bridge/wm_shell_mus.cc
index e13fbffd63d95fb9001c6973fbd5ea26556ea3b3..f5f37d782304bb55b6c2a180b052e3626d33d689 100644
--- a/ash/mus/bridge/wm_shell_mus.cc
+++ b/ash/mus/bridge/wm_shell_mus.cc
@@ -9,6 +9,7 @@
 #include "ash/common/accelerators/accelerator_controller.h"
 #include "ash/common/display/display_info.h"
 #include "ash/common/keyboard/keyboard_ui.h"
+#include "ash/common/scoped_root_window_for_new_windows.h"
 #include "ash/common/session/session_state_delegate.h"
 #include "ash/common/shell_delegate.h"
 #include "ash/common/shell_observer.h"
@@ -211,6 +212,12 @@ WmWindow* WmShellMus::GetRootWindowForNewWindows() {
   return root_window_controllers_[0]->GetWindow();
 }
 
+std::unique_ptr<ScopedRootWindowForNewWindows>
+WmShellMus::CreateScopedRootWindowForNewWindows(WmWindow* root) {
+  NOTIMPLEMENTED();
+  return nullptr;
+}
+
 const DisplayInfo& WmShellMus::GetDisplayInfo(int64_t display_id) const {
   NOTIMPLEMENTED();
   static DisplayInfo fake_info;