- 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
const std::string& message) {
  
  if (!embedder_message_dispatcher_) {
    return;
  }
  std::string method;
  base::ListValue empty_params;
  base::ListValue* params = &empty_params;
  base::DictionaryValue* dict = nullptr;
  std::unique_ptr<base::Value> parsed_message(
      base::JSONReader::ReadDeprecated(message));
  if (!parsed_message || !parsed_message->GetAsDictionary(&dict) ||
      !dict->GetString(kFrontendHostMethod, &method) ||
      (dict->HasKey(kFrontendHostParams) &&
       !dict->GetList(kFrontendHostParams, &params))) {
    LOG(ERROR) << "Invalid message was sent to embedder: " << message;
    return;
  }
  int id = 0;
  dict->GetInteger(kFrontendHostId, &id);
  embedder_message_dispatcher_->Dispatch(
      base::BindRepeating(&InspectableWebContentsImpl::SendMessageAck,
                          weak_factory_.GetWeakPtr(), id),
      method, params);
}
void InspectableWebContentsImpl::DispatchProtocolMessage(