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
mojom::ElectronBrowserRequest request,
    content::RenderFrameHost* render_frame_host) {
  auto id = bindings_.AddBinding(this, std::move(request), render_frame_host);
  frame_to_bindings_map_[render_frame_host].push_back(id);
}
void Ws::OnElectronBrowserConnectionError() {
  auto binding_id = bindings_.dispatch_binding();
  auto* frame_host = bindings_.dispatch_context();
  base::Erase(frame_to_bindings_map_[frame_host], binding_id);
}
void Ws::Message(bool internal,