mac: Refer to NSEvent scrollingDeltaY as a method.

chrome/browser/renderer_host/chrome_render_widget_host_view_mac_history_swiper.mm

Index: chrome/browser/renderer_host/chrome_render_widget_host_view_mac_history_swiper.mm
diff --git a/chrome/browser/renderer_host/chrome_render_widget_host_view_mac_history_swiper.mm b/chrome/browser/renderer_host/chrome_render_widget_host_view_mac_history_swiper.mm
index 2f72c34971a81357946741d61d684020d56519db..b161b84fa8abd0c8bafbab8f12e6d812fe2344fd 100644
--- a/chrome/browser/renderer_host/chrome_render_widget_host_view_mac_history_swiper.mm
+++ b/chrome/browser/renderer_host/chrome_render_widget_host_view_mac_history_swiper.mm
@@ -597,7 +597,7 @@ BOOL forceMagicMouse = NO;
       // It is unclear whether the user is attempting to perform history
       // swiping.  If the event has a vertical component, send it on to the
       // renderer.
-      return event.scrollingDeltaY == 0;
+      return [event scrollingDeltaY] == 0;
   }
 }