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
else if (name == "documents")
    return chrome::DIR_USER_DOCUMENTS;
  else if (name == "downloads")
    return chrome::DIR_DEFAULT_DOWNLOADS;
  else if (name == "music")
    return chrome::DIR_USER_MUSIC;
  else if (name == "pictures")
    return chrome::DIR_USER_PICTURES;
  else if (name == "videos")
    return chrome::DIR_USER_VIDEOS;
  else
    return -1;
}
bool NotificationCallbackWrapper(
    const ProcessSingleton::NotificationCallback& callback,
    const base::CommandLine::StringVector& cmd,
    const base::FilePath& cwd) {
  
  if (Browser::Get()->is_ready()) {
    callback.Run(cmd, cwd);
  } else {
    scoped_refptr<base::SingleThreadTaskRunner> task_runner(
        base::ThreadTaskRunnerHandle::Get());
    task_runner->PostTask(
        FROM_HERE, base::Bind(base::IgnoreResult(callback), cmd, cwd));
  }
  
  return !Browser::Get()->is_shutting_down();
}
void OnClientCertificateSelected(