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
v8::Local<v8::Promise> Ws::SavePage(
    const base::FilePath& full_file_path,
    const content::SavePageType& save_type) {
  gin_helper::Promise<void> promise(isolate());
  v8::Local<v8::Promise> handle = promise.GetHandle();
  auto* handler = new SavePageHandler(ontents(), std::move(promise));
  handler->Handle(full_file_path, save_type);
  return handle;
}
void Ws::OpenDevTools(gin_helper::Arguments* args) {
  if (type_ == Type::REMOTE)
    return;
  if (!enable_devtools_)
    return;
  std::string state;
  if (type_ == Type::WEB_VIEW || !owner_window()) {
    state = "detach";
  }
  bool activate = true;
  if (args && args->Length() == 1) {
    gin_helper::Dictionary options;
    if (args->GetNext(&options)) {
      options.Get("mode", &state);
      options.Get("activate", &activate);
    }
  }
  managed_ontents()->SetDockState(state);
  managed_ontents()->ShowDevTools(activate);
}
void Ws::CloseDevTools() {