- 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/common/platform_util_win.cc
base::ScopedBlockingCall scoped_blocking_call(FROM_HERE,
                                                base::BlockingType::MAY_BLOCK);
  base::win::ScopedCOMInitializer com_initializer;
  if (!com_initializer.Succeeded())
    return;
  base::FilePath dir = full_path.DirName().AsEndingWithSeparator();
  
  if (dir.empty())
    return;
  Microsoft::WRL::ComPtr<IShellFolder> desktop;
  HRESULT hr = SHGetDesktopFolder(desktop.GetAddressOf());
  if (FAILED(hr))
    return;
  base::win::ScopedCoMem<ITEMIDLIST> dir_item;
  hr = desktop->ParseDisplayName(NULL, NULL,
                                 const_cast<wchar_t*>(dir.value().c_str()),
                                 NULL, &dir_item, NULL);
  if (FAILED(hr)) {
    ui::win::OpenFolderViaShell(dir);
    return;
  }
  base::win::ScopedCoMem<ITEMIDLIST> file_item;
  hr = desktop->ParseDisplayName(
      NULL, NULL, const_cast<wchar_t*>(full_path.value().c_str()), NULL,
      &file_item, NULL);