mac: Refer to NSEvent scrollingDeltaY as a method.

Prior to OSX 10.10, the declaration is a method. In OSX 10.10, the declaration
became a property. For simplicity and consistent, always refer to
scrollingDeltaY as a method.

BUG=471823

Review URL: https://codereview.chromium.org/1116563002

Cr-Commit-Position: refs/heads/master@{#327437}

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;
   }
 }