For Mac, add AppListViewDelegateObserver::OnShutdown

ui/app_list/cocoa/app_list_view_controller.mm

Index: ui/app_list/cocoa/app_list_view_controller.mm
diff --git a/ui/app_list/cocoa/app_list_view_controller.mm b/ui/app_list/cocoa/app_list_view_controller.mm
index 0ca22f46a0885c5e517d6f144bd30208edd8497f..b831f9a916126523f805e2c750350cf504261d53 100644
--- a/ui/app_list/cocoa/app_list_view_controller.mm
+++ b/ui/app_list/cocoa/app_list_view_controller.mm
@@ -88,6 +88,7 @@ class AppListModelObserverBridge : public AppListViewDelegateObserver {
  private:
   // Overridden from app_list::AppListViewDelegateObserver:
   virtual void OnProfilesChanged() OVERRIDE;
+  virtual void OnShutdown() OVERRIDE;
 
   AppListViewController* parent_;  // Weak. Owns us.
 
@@ -108,6 +109,10 @@ void AppListModelObserverBridge::OnProfilesChanged() {
   [parent_ onProfilesChanged];
 }
 
+void AppListModelObserverBridge::OnShutdown() {
+  [parent_ setDelegate:nil];
+}
+
 }  // namespace app_list
 
 @implementation AppListViewController