Reland: mus: Enable system modal windows

components/mus/ws/window_tree.cc

Index: components/mus/ws/window_tree.cc
diff --git a/components/mus/ws/window_tree.cc b/components/mus/ws/window_tree.cc
index 0114bf5e89484c2ea0a5aa308e023c9861395187..31c1e9331dfa91e588bc4cdf55986da7f31a9086 100644
--- a/components/mus/ws/window_tree.cc
+++ b/components/mus/ws/window_tree.cc
@@ -242,8 +242,15 @@ bool WindowTree::AddTransientWindow(const ClientWindowId& window_id,
 bool WindowTree::SetModal(const ClientWindowId& window_id) {
   ServerWindow* window = GetWindowByClientId(window_id);
   if (window && access_policy_->CanSetModal(window)) {
-    window->SetModal();
     WindowManagerState* wms = GetWindowManagerState(window);
+    if (window->transient_parent()) {
+      window->SetModal();
+    } else if (user_id_ != InvalidUserId()) {
+      if (wms)
+        wms->AddSystemModalWindow(window);
+    } else {
+      return false;
+    }
     if (wms)
       wms->ReleaseCaptureBlockedByModalWindow(window);
     return true;