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
base::string16 final_title;
  bool explicit_set = true;
  if (entry) {
    auto title = entry->GetTitle();
    auto url = entry->GetURL();
    if (url.SchemeIsFile() && title.empty()) {
      final_title = base::UTF8ToUTF16(url.ExtractFileName());
      explicit_set = false;
    } else {
      final_title = title;
    }
  }
  for (ExtendedWsObserver& observer : observers_)
    observer.OnPageTitleUpdated(final_title, explicit_set);
  Emit("page-title-updated", final_title, explicit_set);
}
void Ws::DidUpdateFaviconURL(
    const std::vector<content::FaviconURL>& urls) {
  std::set<GURL> unique_urls;
  for (const auto& iter : urls) {
    if (iter.icon_type != content::FaviconURL::IconType::kFavicon)
      continue;
    const GURL& url = iter.icon_url;
    if (url.is_valid())
      unique_urls.insert(url);
  }
  Emit("page-favicon-updated", unique_urls);
}
void Ws::DevToolsReloadPage() {
  Emit("devtools-reload-page");