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 863a3a13e61a2095581b0f08fac3b089e39daa76..ea327d49649c3d4d1449d721ba2568e58b39b900 100644
--- a/components/mus/ws/window_tree.cc
+++ b/components/mus/ws/window_tree.cc
@@ -238,14 +238,20 @@ bool WindowTree::AddTransientWindow(const ClientWindowId& window_id,
 
 bool WindowTree::SetModal(const ClientWindowId& window_id) {
   ServerWindow* window = GetWindowByClientId(window_id);
+  bool success = false;
   if (window && access_policy_->CanSetModal(window)) {
-    window->SetModal();
     WindowManagerState* wms = GetWindowManagerState(window);
-    if (wms)
+    if (window->transient_parent()) {
+      window->SetModal();
+      success = true;
+    } else {
+      success = user_id_ != InvalidUserId() && wms &&
+                wms->SetSystemModalWindow(window);

[sky, 2016/04/20 20:14:12]

This is problematic because you're assuming there is only ever one system modal
window. Lets say a client creates two system modal windows, A and B, and then
shows A. I believe that won't work well with how you have the code.

[mohsen, 2016/04/21 17:58:43]

What would exactly not work in your example? The client creates A as system
modal, then creates B and tries to set it as system modal, but it fails and its
modality gets reverted. A remains system modal, B remains a regular window.

+    }
+    if (success)
       wms->ReleaseCaptureBlockedByModalWindow(window);
-    return true;
   }
-  return false;
+  return success;
 }
 
 std::vector<const ServerWindow*> WindowTree::GetWindowTree(