mash: Port mojo:ash_sysui's ContextMenuMus to mojo:ash.

ash/mus/shell_delegate_mus.h

Index: ash/mus/shell_delegate_mus.h
diff --git a/ash/mus/shell_delegate_mus.h b/ash/mus/shell_delegate_mus.h
index 233aa9fb0a5306359110978ecc0bef3d3c24decf..94de2d73e9502476b3ade69c6232465811166319 100644
--- a/ash/mus/shell_delegate_mus.h
+++ b/ash/mus/shell_delegate_mus.h
@@ -41,7 +41,7 @@ class ShellDelegateMus : public ShellDelegate {
   app_list::AppListPresenter* GetAppListPresenter() override;
   ShelfDelegate* CreateShelfDelegate(ShelfModel* model) override;
   SystemTrayDelegate* CreateSystemTrayDelegate() override;
-  UserWallpaperDelegate* CreateUserWallpaperDelegate() override;
+  WallpaperDelegate* CreateWallpaperDelegate() override;
   SessionStateDelegate* CreateSessionStateDelegate() override;
   AccessibilityDelegate* CreateAccessibilityDelegate() override;
   NewWindowDelegate* CreateNewWindowDelegate() override;