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::Rect rect;
  gin_helper::Promise<gfx::Image> promise(isolate());
  v8::Local<v8::Promise> handle = promise.GetHandle();
  // get rect arguments if they exist
  args->GetNext(&rect);
  auto* const view = ontents()->GetRenderWidgetHostView();
  if (!view) {
    promise.Resolve(gfx::Image());
    return handle;
  }
  // Capture full page if user doesn't specify a |rect|.
  const gfx::Size view_size =
      rect.IsEmpty() ? view->GetViewBounds().size() : rect.size();
  // By default, the requested bitmap size is the view size in screen
  // coordinates.  However, if there's more pixel detail available on the
  // current system, increase the requested bitmap size to capture it all.
  gfx::Size bitmap_size = view_size;
  const gfx::NativeView native_view = view->GetNativeView();
  const float scale = display::Screen::GetScreen()
                          ->GetDisplayNearestView(native_view)
                          .device_scale_factor();
  if (scale > 1.0f)
    bitmap_size = gfx::ScaleToCeiledSize(view_size, scale);
  view->CopyFromSurface(gfx::Rect(rect.origin(), view_size), bitmap_size,
                        base::BindOnce(&OnCapturePageDone, std::move(promise)));
  return handle;
}
void Ws::IncrementCapturerCount(gin_helper::Arguments* args) {