- 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,
    const std::string& message) {
  if (!frontend_loaded_)
    return;
  if (message.length() < kMaxMessageChunkSize) {
    base::string16 javascript =
        base::UTF8ToUTF16("DevToolsAPI.dispatchMessage(" + message + ");");
    GetDevToolsWebContents()->GetMainFrame()->ExecuteJavaScript(
        javascript, base::NullCallback());
    return;
  }
  base::Value total_size(static_cast<int>(message.length()));
  for (size_t pos = 0; pos < message.length(); pos += kMaxMessageChunkSize) {
    base::Value message_value(message.substr(pos, kMaxMessageChunkSize));
    CallClientFunction("DevToolsAPI.dispatchMessageChunk", &message_value,
                       pos ? nullptr : &total_size, nullptr);
  }
}
void InspectableWebContentsImpl::AgentHostClosed(