Use a single ToplevelWindowEventHandler for the ash::Shell

ash/shell.h

Index: ash/shell.h
diff --git a/ash/shell.h b/ash/shell.h
index 4b8ab2172437b729ff2d7785b681306f19ab44b7..d1a427931d57a1b96ce74ef3e94f802b6fdd696b 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -105,6 +105,7 @@ class StickyKeys;
 class SystemTray;
 class SystemTrayDelegate;
 class SystemTrayNotifier;
+class ToplevelWindowEventHandler;
 class UserActivityDetector;
 class UserWallpaperDelegate;
 class VideoDetector;
@@ -655,6 +656,9 @@ class ASH_EXPORT Shell
   // An event filter for logging keyboard-related metrics.
   scoped_ptr<internal::KeyboardUMAEventFilter> keyboard_metrics_filter_;
 
+  // An event filter which handles moving and resizing windows.
+  scoped_ptr<ToplevelWindowEventHandler> toplevel_window_event_handler_;
+
   // An event filter which handles system level gestures
   scoped_ptr<internal::SystemGestureEventFilter> system_gesture_filter_;