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
if (imported_certs.size() > 1) {
        auto it = imported_certs.begin();
        ++it;  
        for (; it != imported_certs.end(); ++it)
          rv &= model->SetCertTrust(it->get(),
                                    net::CA_CERT,
                                    net::NSSCertDatabase::TRUSTED_SSL);
      }
    }
  }
  return rv;
}
#endif
}  
App::App(v8::Isolate* isolate) {
  static_cast<AtomBrowserClient*>(AtomBrowserClient::Get())->set_delegate(this);
  Browser::Get()->AddObserver(this);
  content::GpuDataManager::GetInstance()->AddObserver(this);
  Init(isolate);
}
App::~App() {
  static_cast<AtomBrowserClient*>(AtomBrowserClient::Get())->set_delegate(
      nullptr);
  Browser::Get()->RemoveObserver(this);
  content::GpuDataManager::GetInstance()->RemoveObserver(this);
}
void App::OnBeforeQuit(bool* prevent_default) {
  *prevent_default = Emit("before-quit");
}
void App::OnWillQuit(bool* prevent_default) {