- 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
NetworkResourceLoader& operator=(const NetworkResourceLoader&) = delete;
 private:
  void OnResponseStarted(const GURL& final_url,
                         const network::mojom::URLResponseHead& response_head) {
    response_headers_ = response_head.headers;
  }
  void OnDataReceived(base::StringPiece chunk,
                      base::OnceClosure resume) override {
    base::Value chunkValue;
    bool encoded = !base::IsStringUTF8(chunk);
    if (encoded) {
      std::string encoded_string;
      base::Base64Encode(chunk, &encoded_string);
      chunkValue = base::Value(std::move(encoded_string));
    } else {
      chunkValue = base::Value(chunk);
    }
    base::Value id(stream_id_);
    base::Value encodedValue(encoded);
    bindings_->CallClientFunction("DevToolsAPI.streamWrite", &id, &chunkValue,
                                  &encodedValue);
    std::move(resume).Run();
  }
  void OnComplete(bool success) override {