Tab dragging as implemented as a mus API.

ui/views/mus/native_widget_mus.h

Index: ui/views/mus/native_widget_mus.h
diff --git a/ui/views/mus/native_widget_mus.h b/ui/views/mus/native_widget_mus.h
index 33da5378358e1cc3cbb3d1b975ea63e7f065474c..93d7180f58cf5a28338ce5aba255b8df9883bfe4 100644
--- a/ui/views/mus/native_widget_mus.h
+++ b/ui/views/mus/native_widget_mus.h
@@ -264,6 +264,11 @@ private:
   // a separate class.
   std::unique_ptr<MusWindowObserver> mus_window_observer_;
 
+  // This is misnamed; The native widget interface offers something called
+  // "native window properties" which are properties which it stores locally,
+  // and this is used to unsafely pass void* pointers around chrome.
+  std::map<std::string, void*> native_window_properties_;
+
   // Aura configuration.
   std::unique_ptr<SurfaceContextFactory> context_factory_;
   std::unique_ptr<WindowTreeHostMus> window_tree_host_;