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
if (!enable_devtools_)
    return;
  for (const auto& agent_host : content::DevToolsAgentHost::GetOrCreateAll()) {
    if (agent_host->GetType() ==
        content::DevToolsAgentHost::kTypeSharedWorker) {
      OpenDevTools(nullptr);
      managed_ontents()->AttachTo(agent_host);
      break;
    }
  }
}
void Ws::InspectServiceWorker() {
  if (type_ == Type::REMOTE)
    return;
  if (!enable_devtools_)
    return;
  for (const auto& agent_host : content::DevToolsAgentHost::GetOrCreateAll()) {
    if (agent_host->GetType() ==
        content::DevToolsAgentHost::kTypeServiceWorker) {
      OpenDevTools(nullptr);
      managed_ontents()->AttachTo(agent_host);
      break;
    }
  }
}
void Ws::SetIgnoreMenuShortcuts(bool ignore) {
  auto* web_preferences = WsPreferences::From(ontents());
  DCHECK(web_preferences);
  web_preferences->preference()->SetKey("ignoreMenuShortcuts",
                                        base::Value(ignore));