Electron Version: 7.1.1
Operating System: Windows 10 (1803)

If I set the 'margins' option when call webContents.print method,
The app will crash with no error message.

```js
let { remote } = require("electron");
let webContents = remote.getCurrentWebContents();
webContents.print(
  {
    margins: {
      marginType: "custom",
      top: 30,
      bottom: 0,
      left: 0,
      right: 0
    }
  },
  (success, errorType) => {
    if (!success) console.log(errorType);
  }
);
```

shell/browser/api/electron_api_web_contents.cc
int object_id,
                                            int ref_count) {
  base::ListValue args;
  args.Append(context_id);
  args.Append(object_id);
  args.Append(ref_count);
  EmitWithSender("-ipc-message", bindings_.dispatch_context(), InvokeCallback(),
                 /* interELECTRON_BROWSER_DEREFERENCE",
                 std::move(args));
}
#endif
void Ws::UpdateDraggableRegions(
    std::vector<mojom::DraggableRegionPtr> regions) {
  for (ExtendedWsObserver& observer : observers_)
    observer.OnDraggableRegionsUpdated(regions);
}
void Ws::RenderFrameDeleted(
    content::RenderFrameHost* render_frame_host) {
  // A RenderFrameHost can be destroyed before the related Mojo binding is
  // closed, which can result in Mojo calls being sent for RenderFrameHosts
  // that no longer exist. To prevent this from happening, when a
  // RenderFrameHost goes away, we close all the bindings related to that
  // frame.
  auto it = frame_to_bindings_map_.find(render_frame_host);
  if (it == frame_to_bindings_map_.end())
    return;
  for (auto id : it->second)
    bindings_.RemoveBinding(id);
  frame_to_bindings_map_.erase(it);