[osx] Add mouse coordinates to os::Event::DropFiles event indicating …

…the point where the drop of files took place on the target window

os/osx/view.mm

@@ -661,6 +661,9 @@ - (BOOL)performDragOperation:(id <NSDraggingInfo>)sender
     NSArray* filenames = [pasteboard propertyListForType:NSFilenamesPboardType];
 
     os::Event ev = generate_drop_files_from_nsarray(filenames);
+    ev.setPosition(gfx::Point(
+      sender.draggingLocation.x,
+      [sender.draggingDestinationWindow contentRectForFrameRect: sender.draggingDestinationWindow.frame].size.height - sender.draggingLocation.y));
     [self queueEvent:ev];
     return YES;
   }