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
if (app_user_model_id_.empty()) {
    SetAppUserModelID(base::ReplaceStringPlaceholders(
        kAppUserModelIDFormat, base::UTF8ToUTF16(GetName()), nullptr));
  }
  return app_user_model_id_.c_str();
}
std::string Browser::GetExecutableFileVersion() const {
  base::FilePath path;
  if (PathService::Get(base::FILE_EXE, &path)) {
    scoped_ptr<FileVersionInfo> version_info(
        FileVersionInfo::CreateFileVersionInfo(path));
    return base::UTF16ToUTF8(version_info->product_version());
  }
  return ATOM_VERSION_STRING;
}
std::string Browser::GetExecutableFileProductName() const {
  base::FilePath path;
  if (PathService::Get(base::FILE_EXE, &path)) {
    scoped_ptr<FileVersionInfo> version_info(
        FileVersionInfo::CreateFileVersionInfo(path));
    return base::UTF16ToUTF8(version_info->product_name());
  }
  return ATOM_PRODUCT_NAME;
}
}