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 14e6a6e7895fb81f6a350a09f23a2a83e657ef18..3fd3f453757595b689f5ce4afc3cc86389323b44 100644
--- a/components/mus/ws/window_tree.cc
+++ b/components/mus/ws/window_tree.cc
@@ -234,8 +234,7 @@ bool WindowTree::AddTransientWindow(const ClientWindowId& window_id,
   if (window && transient_window && !transient_window->Contains(window) &&
       access_policy_->CanAddTransientWindow(window, transient_window)) {
     Operation op(this, window_server_, OperationType::ADD_TRANSIENT_WINDOW);
-    window->AddTransientWindow(transient_window);
-    return true;
+    return window->AddTransientWindow(transient_window);
   }
   return false;
 }
@@ -243,8 +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;