- 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
frontend_loaded_ = false;
  external_devtools_web_contents_ = nullptr;
  Observe(nullptr);
  Detach();
  embedder_message_dispatcher_.reset();
  if (view_ && view_->GetDelegate())
    view_->GetDelegate()->DevToolsClosed();
}
bool InspectableWebContentsImpl::DidAddMessageToConsole(
    content::WebContents* source,
    blink::mojom::ConsoleMessageLevel level,
    const base::string16& message,
    int32_t line_no,
    const base::string16& source_id) {
  logging::LogMessage("CONSOLE", line_no,
                      blink::ConsoleMessageLevelToLogSeverity(level))
          .stream()
      << "\"" << message << "\", source: " << source_id << " (" << line_no
      << ")";
  return true;
}
bool InspectableWebContentsImpl::HandleKeyboardEvent(
    content::WebContents* source,
    const content::NativeWebKeyboardEvent& event) {
  auto* delegate = web_contents_->GetDelegate();
  return !delegate || delegate->HandleKeyboardEvent(source, event);
}
void InspectableWebContentsImpl::CloseContents(content::WebContents* source) {