- 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
return S_OK;
}
HRESULT DeleteFileProgressSink::ResetTimer() {
  return S_OK;
}
HRESULT DeleteFileProgressSink::PauseTimer() {
  return S_OK;
}
HRESULT DeleteFileProgressSink::ResumeTimer() {
  return S_OK;
}
std::string OpenExternalOnWorkerThread(
    const GURL& url,
    const platform_util::OpenExternalOptions& options) {
  base::ScopedBlockingCall scoped_blocking_call(FROM_HERE,
                                                base::BlockingType::MAY_BLOCK);
  
  
  
  base::string16 escaped_url = L"\"" + base::UTF8ToUTF16(url.spec()) + L"\"";
  base::string16 working_dir = options.working_dir.value();
  if (reinterpret_cast<ULONG_PTR>(
          ShellExecuteW(nullptr, L"open", escaped_url.c_str(), nullptr,
                        working_dir.empty() ? nullptr : working_dir.c_str(),
                        SW_SHOWNORMAL)) <= 32) {
    return "Failed to open";
  }
  return "";
}
void ShowItemInFolderOnWorkerThread(const base::FilePath& full_path) {