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
frame_subscriber_.reset();
}
void Ws::StartDrag(const gin_helper::Dictionary& item,
                            gin_helper::Arguments* args) {
  base::FilePath file;
  std::vector<base::FilePath> files;
  if (!item.Get("files", &files) && item.Get("file", &file)) {
    files.push_back(file);
  }
  gin::Handle<NativeImage> icon;
  if (!item.Get("icon", &icon)) {
    args->ThrowError("Must specify non-empty 'icon' option");
    return;
  }
  // Start dragging.
  if (!files.empty()) {
    base::MessageLoopCurrent::ScopedNestableTaskAllower allow;
    DragFileItems(files, icon->image(), ontents()->GetNativeView());
  } else {
    args->ThrowError("Must specify either 'file' or 'files' option");
  }
}
v8::Local<v8::Promise> Ws::CapturePage(gin_helper::Arguments* args) {