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
gfx::Size size;
  bool stay_hidden = false;
  // get size arguments if they exist
  args->GetNext(&size);
  // get stayHidden arguments if they exist
  args->GetNext(&stay_hidden);
  ontents()->IncrementCapturerCount(size, stay_hidden);
}
void Ws::DecrementCapturerCount(gin_helper::Arguments* args) {
  bool stay_hidden = false;
  // get stayHidden arguments if they exist
  args->GetNext(&stay_hidden);
  ontents()->DecrementCapturerCount(stay_hidden);
}
bool Ws::IsBeingCaptured() {
  return ontents()->IsBeingCaptured();
}
void Ws::OnCursorChange(const content::WebCursor& cursor) {
  const content::CursorInfo& info = cursor.info();
  if (info.type == ui::CursorType::kCustom) {
    Emit("cursor-changed", CursorTypeToString(info),
         gfx::Image::CreateFrom1xBitmap(info.custom_image),
         info.image_scale_factor,
         gfx::Size(info.custom_image.width(), info.custom_image.height()),
         info.hotspot);
  } else {
    Emit("cursor-changed", CursorTypeToString(info));
  }
}
bool Ws::IsGuest() const {