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/browser_win.cc
return true;
  std::wstring keyVal;
  if (FAILED(commandKey.ReadValue(L"", &keyVal)))
    
    return true;
  std::wstring exePath(path.value());
  std::wstring exe = L"\"" + exePath + L"\" \"%1\"";
  if (keyVal == exe) {
    
    if (FAILED(key.DeleteKey(L"shell")))
      return false;
    return true;
  } else {
    return true;
  }
}
bool Browser::SetAsDefaultProtocolClient(const std::string& protocol) {
  
  
  
  
  
  
  
  
  
  
   Administrator user. So, we instead write to "HKEY_CURRENT_USER\
   Software\Classes", which is inherited by "HKEY_CLASSES_ROOT"