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
auto result = SessionPreferences::GetValidPreloads(GetBrowserContext());
  if (auto* web_preferences = WsPreferences::From(ontents())) {
    base::FilePath::StringType preload;
    if (web_preferences->GetPreloadPath(&preload)) {
      result.emplace_back(preload);
    }
  }
  return result;
}
v8::Local<v8::Value> Ws::GetWebPreferences(
    v8::Isolate* isolate) const {
  auto* web_preferences = WsPreferences::From(ontents());
  if (!web_preferences)
    return v8::Null(isolate);
  return gin::ConvertToV8(isolate, *web_preferences->preference());
}
v8::Local<v8::Value> Ws::GetLastWebPreferences(
    v8::Isolate* isolate) const {
  auto* web_preferences = WsPreferences::From(ontents());
  if (!web_preferences)
    return v8::Null(isolate);
  return gin::ConvertToV8(isolate, *web_preferences->last_preference());
}
v8::Local<v8::Value> Ws::GetOwnerBrowserWindow() const {
  if (owner_window())
    return BrowserWindow::From(isolate(), owner_window());
  else
    return v8::Null(isolate());
}
int32_t Ws::ID() const {