Moves management of transients out of Window

ash/wm/mru_window_tracker.cc

Index: ash/wm/mru_window_tracker.cc
diff --git a/ash/wm/mru_window_tracker.cc b/ash/wm/mru_window_tracker.cc
index 4ed9a1df7230add4d26cfe61e7f7828bd220b76a..af703e9aa12aba161aa304b6ce1a30f596b5aa12 100644
--- a/ash/wm/mru_window_tracker.cc
+++ b/ash/wm/mru_window_tracker.cc
@@ -190,7 +190,10 @@ void MruWindowTracker::OnWindowActivated(aura::Window* gained_active,
     SetActiveWindow(gained_active);
 }
 
-void MruWindowTracker::OnWindowDestroying(aura::Window* window) {
+void MruWindowTracker::OnWindowDestroyed(aura::Window* window) {
+  // It's possible for OnWindowActivated() to be called after
+  // OnWindowDestroying(). This means we need to override OnWindowDestroyed()
+  // else we may end up with a deleted window in |mru_windows_|.
   mru_windows_.remove(window);
   window->RemoveObserver(this);
 }