The new `setAsDefaultProtocolClient`  and `removeAsDefaultProtocolClient` APIs are great, but without a way to query the current state, it's not possible to implement a UI element like this one. I think it'd be great to add `isDefaultProtocolClient`!

![image](https://cloud.githubusercontent.com/assets/1037212/14545184/fc1a6fac-026a-11e6-98d5-8bf6b2f9678e.png)

Also, I find the naming of these APIs pretty confusing. On Mac OS X and iOS `mailto` is referred to as a "URL Scheme", not a protocol (https://developer.apple.com/library/ios/featuredarticles/iPhoneURLScheme_Reference), and it seems like that might be the case on Windows too. (https://msdn.microsoft.com/en-us/library/aa767914(v=vs.85).aspx)

It'd be nice to clarify that this is registering the app system-wide, while the other `protocol` module is only for handling requests initiated from within the app. 

Maybe we could change these to `registerForURLScheme`, `isRegisteredForURLScheme`?

atom/browser/api/atom_api_app.cc
auto browser_context = static_cast<AtomBrowserContext*>(
        AtomBrowserMainParts::Get()->browser_context());
  browser_context->AllowNTLMCredentialsForAllDomains(should_allow);
}
std::string App::GetLocale() {
  return l10n_util::GetApplicationLocale("");
}
bool App::MakeSingleInstance(
    const ProcessSingleton::NotificationCallback& callback) {
  if (process_singleton_.get())
    return false;
  base::FilePath user_dir;
  PathService::Get(brightray::DIR_USER_DATA, &user_dir);
  process_singleton_.reset(new ProcessSingleton(
      user_dir, base::Bind(NotificationCallbackWrapper, callback)));
  switch (process_singleton_->NotifyOtherProcessOrCreate()) {
    case ProcessSingleton::NotifyResult::LOCK_ERROR:
    case ProcessSingleton::NotifyResult::PROFILE_IN_USE:
    case ProcessSingleton::NotifyResult::PROCESS_NOTIFIED:
      process_singleton_.reset();
      return true;
    case ProcessSingleton::NotifyResult::PROCESS_NONE:
    default:  
      return false;
  }
}
#if defined(USE_NSS_CERTS)
void App::ImportCertificate(