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
const GURL& validated_url) {
  bool is_main_frame = !render_frame_host->GetParent();
  int frame_process_id = render_frame_host->GetProcess()->GetID();
  int frame_routing_id = render_frame_host->GetRoutingID();
  auto weak_this = GetWeakPtr();
  Emit("did-frame-finish-load", is_main_frame, frame_process_id,
       frame_routing_id);
  // ⚠️WARNING!⚠️
  // Emit() triggers JS which can call destroy() on |this|. It's not safe to
  // assume that |this| points to valid memory at this point.
  if (is_main_frame && weak_this)
    Emit("did-finish-load");
}
void Ws::DidFailLoad(content::RenderFrameHost* render_frame_host,
                              const GURL& url,
                              int error_code) {
  bool is_main_frame = !render_frame_host->GetParent();
  int frame_process_id = render_frame_host->GetProcess()->GetID();
  int frame_routing_id = render_frame_host->GetRoutingID();
  Emit("did-fail-load", error_code, "", url, is_main_frame, frame_process_id,
       frame_routing_id);
}
void Ws::DidStartLoading() {
  Emit("did-start-loading");
}
void Ws::DidStopLoading() {