Check that child window was not destroyed inside UpdateTransientChildVisibility.

ui/wm/core/transient_window_manager.cc

Index: ui/wm/core/transient_window_manager.cc
diff --git a/ui/wm/core/transient_window_manager.cc b/ui/wm/core/transient_window_manager.cc
index 2ca145d2420dbdd9f9a2fc81fd785e8e64d7a495..f9d723fb0af05f4b1281eef11532ce655e0f5d9c 100644
--- a/ui/wm/core/transient_window_manager.cc
+++ b/ui/wm/core/transient_window_manager.cc
@@ -11,6 +11,7 @@
 #include "base/stl_util.h"
 #include "ui/aura/window.h"
 #include "ui/aura/window_property.h"
+#include "ui/aura/window_tracker.h"
 #include "ui/wm/core/transient_window_observer.h"
 #include "ui/wm/core/transient_window_stacking_client.h"
 #include "ui/wm/core/window_util.h"
@@ -162,8 +163,17 @@ void TransientWindowManager::OnWindowVisibilityChanged(Window* window,
 
   // If the window has transient children, updates the transient children's
   // visiblity as well.
+  // WindowTracker is used because child window
+  // could be deleted inside UpdateTransientChildVisibility call.
+  aura::WindowTracker tracker;
   for (Window* child : transient_children_)
-    Get(child)->UpdateTransientChildVisibility(visible);
+    tracker.Add(child);
+
+  while (!tracker.windows().empty()) {
+    Window* window = *(tracker.windows().begin());
+    Get(window)->UpdateTransientChildVisibility(visible);
+    tracker.Remove(window);
+  }
 
   // Remember the show request in |show_on_parent_visible_| and hide it again
   // if the following conditions are met