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
content::WebContents* web_contents,
    net::SSLCertRequestInfo* cert_request_info,
    scoped_ptr<content::ClientCertificateDelegate> delegate) {
  std::shared_ptr<content::ClientCertificateDelegate>
      shared_delegate(delegate.release());
  bool prevent_default =
      Emit("select-client-certificate",
           WebContents::CreateFrom(isolate(), web_contents),
           cert_request_info->host_and_port.ToString(),
           cert_request_info->client_certs,
           base::Bind(&OnClientCertificateSelected,
                      isolate(),
                      shared_delegate));
  
  if (!prevent_default)
    shared_delegate->ContinueWithCertificate(
        cert_request_info->client_certs[0].get());
}
void App::OnGpuProcessCrashed(base::TerminationStatus exit_code) {
  Emit("gpu-process-crashed");
}
base::FilePath App::GetPath(mate::Arguments* args, const std::string& name) {