Reland: mus: Enable system modal windows

components/mus/ws/window_tree.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/mus/ws/window_tree.h"
 
 #include <stddef.h>
 
 #include <utility>
 
@@ skipping 224 matching lines @@
     Operation op(this, window_server_, OperationType::ADD_TRANSIENT_WINDOW);
     window->AddTransientWindow(transient_window);
     return true;
   }
   return false;
 }
 
 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;
   }
   return false;
 }
 
 std::vector<const ServerWindow*> WindowTree::GetWindowTree(
     const ClientWindowId& window_id) const {
   const ServerWindow* window = GetWindowByClientId(window_id);
@@ skipping 1174 matching lines @@
 }
 
 bool WindowTree::IsWindowRootOfAnotherTreeForAccessPolicy(
     const ServerWindow* window) const {
   WindowTree* tree = window_server_->GetTreeWithRoot(window);
   return tree && tree != this;
 }
 
 }  // namespace ws
 }  // namespace mus