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
bool initially_shown = type_ != Type::BROWSER_VIEW;
  options.Get(options::kShow, &initially_shown);
  
  std::string partition;
  gin::Handle<api::Session> session;
  if (options.Get("session", &session) && !session.IsEmpty()) {
  } else if (options.Get("partition", &partition)) {
    session = Session::FromPartition(isolate, partition);
  } else {
    
    session = Session::FromPartition(isolate, "");
  }
  session_.Reset(isolate, session.ToV8());
  std::unique_ptr<content::Ws> ontents;
  if (IsGuest()) {
    scoped_refptr<content::SiteInstance> site_instance =
        content::SiteInstance::CreateForURL(session->browser_context(),
                                            GURL("chrome-guest://fake-host"));
    content::Ws::CreateParams params(session->browser_context(),
                                              site_instance);
    guest_delegate_ =
        std::make_unique<WebViewGuestDelegate>(embedder_->ontents(), this);
    params.guest_delegate = guest_delegate_.get();
#if BUILDFLAG(ENABLE_OSR)
    if (embedder_ && embedder_->IsOffScreen()) {
      auto* view = new OffScreenWsView(
          false,