- 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
content::DevToolsAgentHost* agent_host) {}
void InspectableWebContentsImpl::RenderFrameHostChanged(
    content::RenderFrameHost* old_host,
    content::RenderFrameHost* new_host) {
  if (new_host->GetParent())
    return;
  frontend_host_ = content::DevToolsFrontendHost::Create(
      new_host,
      base::BindRepeating(
          &InspectableWebContentsImpl::HandleMessageFromDevToolsFrontend,
          weak_factory_.GetWeakPtr()));
}
void InspectableWebContentsImpl::WebContentsDestroyed() {