Gets mash to use WmToplevelWindowEventHandler

ash/wm/common/wm_toplevel_window_event_handler.h

Index: ash/wm/common/wm_toplevel_window_event_handler.h
diff --git a/ash/wm/common/wm_toplevel_window_event_handler.h b/ash/wm/common/wm_toplevel_window_event_handler.h
index 832cc7742738f3a202f7f6a335230b2b165a076a..fb2626c576c40a762aa365ed2e0eaf4ef327456c 100644
--- a/ash/wm/common/wm_toplevel_window_event_handler.h
+++ b/ash/wm/common/wm_toplevel_window_event_handler.h
@@ -68,6 +68,9 @@ class ASH_WM_COMMON_EXPORT WmToplevelWindowEventHandler
   // If there is a drag in progress it is reverted, otherwise does nothing.
   void RevertDrag();
 
+  // Returns true if there is a drag in progress.
+  bool is_drag_in_progress() const { return window_resizer_.get() != nullptr; }
+
  private:
   class ScopedWindowResizer;