- 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::ShowItemInFolder(
    const std::string& file_system_path) {
  if (file_system_path.empty())
    return;
  base::FilePath path = base::FilePath::FromUTF8Unsafe(file_system_path);
  platform_util::OpenItem(path);
}
void InspectableWebContentsImpl::SaveToFile(const std::string& url,
                                            const std::string& content,
                                            bool save_as) {
  if (delegate_)
    delegate_->DevToolsSaveToFile(url, content, save_as);
}
void InspectableWebContentsImpl::AppendToFile(const std::string& url,
                                              const std::string& content) {
  if (delegate_)
    delegate_->DevToolsAppendToFile(url, content);
}
void InspectableWebContentsImpl::RequestFileSystems() {
  if (delegate_)
    delegate_->DevToolsRequestFileSystems();
}
void InspectableWebContentsImpl::AddFileSystem(const std::string& type) {
  if (delegate_)
    delegate_->DevToolsAddFileSystem(type, base::FilePath());
}
void InspectableWebContentsImpl::RemoveFileSystem(