mash: Migrate ScopedTargetRootWindow to //ash/common

ash/aura/wm_shell_aura.h

Index: ash/aura/wm_shell_aura.h
diff --git a/ash/aura/wm_shell_aura.h b/ash/aura/wm_shell_aura.h
index cd8b7b0291e140fa15ed114a698e8ca7b02ddd8b..743a7390b20c4565758b30418a7e902a8cd460e5 100644
--- a/ash/aura/wm_shell_aura.h
+++ b/ash/aura/wm_shell_aura.h
@@ -34,6 +34,8 @@ class ASH_EXPORT WmShellAura : public WmShell,
   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;