- 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
const DispatchCallback& callback) {
  const base::DictionaryValue* prefs =
      pref_service_->GetDictionary(kDevToolsPreferences);
  callback.Run(prefs);
}
void InspectableWebContentsImpl::SetPreference(const std::string& name,
                                               const std::string& value) {
  DictionaryPrefUpdate update(pref_service_, kDevToolsPreferences);
  update.Get()->SetKey(name, base::Value(value));
}
void InspectableWebContentsImpl::RemovePreference(const std::string& name) {
  DictionaryPrefUpdate update(pref_service_, kDevToolsPreferences);
  update.Get()->RemoveWithoutPathExpansion(name, nullptr);
}
void InspectableWebContentsImpl::ClearPreferences() {
  DictionaryPrefUpdate update(pref_service_, kDevToolsPreferences);
  update.Get()->Clear();
}
void InspectableWebContentsImpl::ConnectionReady() {}
void InspectableWebContentsImpl::RegisterExtensionsAPI(
    const std::string& origin,
    const std::string& script) {
  extensions_api_[origin + "/"] = script;
}
void InspectableWebContentsImpl::HandleMessageFromDevToolsFrontend(