chromeos: wires transients using the WindowTreeHost window

ui/views/mus/desktop_window_tree_host_mus.cc

Index: ui/views/mus/desktop_window_tree_host_mus.cc
diff --git a/ui/views/mus/desktop_window_tree_host_mus.cc b/ui/views/mus/desktop_window_tree_host_mus.cc
index 3dc3c368fceae94bfc548359e6ab2c908d0c8b89..9cff6ce99ca63f8bbdb57d97e12ceb2b82202b7f 100644
--- a/ui/views/mus/desktop_window_tree_host_mus.cc
+++ b/ui/views/mus/desktop_window_tree_host_mus.cc
@@ -272,9 +272,12 @@ void DesktopWindowTreeHostMus::Init(aura::Window* content_window,
   aura::client::SetCursorClient(window(), cursor_manager_.get());
   InitHost();
 
-  if (params.parent) {
-    aura::client::GetTransientWindowClient()->AddTransientChild(params.parent,
-                                                                window());
+  // Transient parents are connected using the Window created by WindowTreeHost,
+  // which is owned by the window manager. This way the window manager can
+  // properly identify and honor transients.
+  if (params.parent && params.parent->GetHost()) {
+    aura::client::GetTransientWindowClient()->AddTransientChild(
+        params.parent->GetHost()->window(), window());
   }
 }