Reland: mus: Enable system modal windows

components/mus/public/cpp/lib/window.cc

Index: components/mus/public/cpp/lib/window.cc
diff --git a/components/mus/public/cpp/lib/window.cc b/components/mus/public/cpp/lib/window.cc
index 1e565bae5277f0506e6c86b9f910dac670a30123..4433fb53ece1a5e0c207868e80980680b2613c3d 100644
--- a/components/mus/public/cpp/lib/window.cc
+++ b/components/mus/public/cpp/lib/window.cc
@@ -343,6 +343,9 @@ bool Window::Contains(const Window* child) const {
 }
 
 void Window::AddTransientWindow(Window* transient_window) {
+  // A system modal window cannot become a transient child.
+  DCHECK(!transient_window->is_modal() || transient_window->transient_parent());
+
   if (connection_)
     CHECK_EQ(transient_window->connection(), connection_);
   LocalAddTransientWindow(transient_window);