- Electron version: 1.3.6
- Operating system: Windows 7 x64
- Other: Microsoft Office 2013 (will make more sense in a bit)

`shell.openItem()` is a blocking call, it would be nice if it's async or has an async version to handle synchronous external application behavior.

Reproduce: 
Open a password-protected Microsoft Excel file, `shell.openItem()` will be blocking until the Password window is submitted or dismissed. Consequently Microsoft Word does not have this issue.

Related to #2761.

shell/browser/ui/inspectable_web_contents_impl.cc
void InspectableWebContentsImpl::InspectedURLChanged(const std::string& url) {
  if (managed_devtools_web_contents_)
    view_->SetTitle(
        base::UTF8ToUTF16(base::StringPrintf(kTitleFormat, url.c_str())));
}
void InspectableWebContentsImpl::LoadNetworkResource(
    const DispatchCallback& callback,
    const std::string& url,
    const std::string& headers,
    int stream_id) {
  GURL gurl(url);
  if (!gurl.is_valid()) {
    base::DictionaryValue response;
    response.SetInteger("statusCode", 404);
    callback.Run(&response);
    return;
  }
  auto resource_request = std::make_unique<network::ResourceRequest>();
  resource_request->url = gurl;
  resource_request->headers.AddHeadersFromString(headers);
  auto* partition = content::BrowserContext::GetDefaultStoragePartition(
      GetDevToolsWebContents()->GetBrowserContext());
  auto factory = partition->GetURLLoaderFactoryForBrowserProcess();
  auto simple_url_loader = network::SimpleURLLoader::Create(
      std::move(resource_request), MISSING_TRAFFIC_ANNOTATION);
  auto resource_loader = std::make_unique<NetworkResourceLoader>(
      stream_id, this, std::move(simple_url_loader), factory.get(), callback);
  loaders_.insert(std::move(resource_loader));
}
void InspectableWebContentsImpl::SetIsDocked(const DispatchCallback& callback,