WIP: Eliminate event.target() usage from TrayEventFilter

ash/shell.h

Index: ash/shell.h
diff --git a/ash/shell.h b/ash/shell.h
index 1db72a6247b226fc2b4461df72ad6654e6013023..4736282f09720cb63489f723d31de1d7e3de3c48 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -88,6 +88,7 @@ class AshNativeCursorManager;
 class AutoclickController;
 class BluetoothNotificationController;
 class CaptureController;
+class ContainerDelegate;
 class DesktopBackgroundController;
 class DisplayChangeObserver;
 class DisplayColorManager;
@@ -442,6 +443,8 @@ class ASH_EXPORT Shell : public SystemModalContainerEventFilterDelegate,
     return media_delegate_.get();
   }
 
+  ContainerDelegate* container_delegate() { return container_delegate_.get(); }
+
   HighContrastController* high_contrast_controller() {
     return high_contrast_controller_.get();
   }
@@ -688,6 +691,7 @@ class ASH_EXPORT Shell : public SystemModalContainerEventFilterDelegate,
   std::unique_ptr<AccessibilityDelegate> accessibility_delegate_;
   std::unique_ptr<NewWindowDelegate> new_window_delegate_;
   std::unique_ptr<MediaDelegate> media_delegate_;
+  std::unique_ptr<ContainerDelegate> container_delegate_;
   std::unique_ptr<PointerWatcherDelegate> pointer_watcher_delegate_;
   std::unique_ptr<ShelfDelegate> shelf_delegate_;
   std::unique_ptr<ShelfItemDelegateManager> shelf_item_delegate_manager_;