- 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& file_system_path) {
  if (delegate_)
    delegate_->DevToolsRemoveFileSystem(
        base::FilePath::FromUTF8Unsafe(file_system_path));
}
void InspectableWebContentsImpl::UpgradeDraggedFileSystemPermissions(
    const std::string& file_system_url) {}
void InspectableWebContentsImpl::IndexPath(
    int request_id,
    const std::string& file_system_path,
    const std::string& excluded_folders) {
  if (delegate_)
    delegate_->DevToolsIndexPath(request_id, file_system_path,
                                 excluded_folders);
}
void InspectableWebContentsImpl::StopIndexing(int request_id) {
  if (delegate_)
    delegate_->DevToolsStopIndexing(request_id);
}
void InspectableWebContentsImpl::SearchInPath(
    int request_id,
    const std::string& file_system_path,
    const std::string& query) {
  if (delegate_)
    delegate_->DevToolsSearchInPath(request_id, file_system_path, query);
}
void InspectableWebContentsImpl::SetWhitelistedShortcuts(