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_linux.cc
WindowList* list = WindowList::GetInstance();
  for (WindowList::iterator iter = list->begin(); iter != list->end(); ++iter) {
    NativeWindow* window = *iter;
    if (window->IsVisible()) {
      window->Focus(true);
      break;
    }
  }
}
void Browser::AddRecentDocument(const base::FilePath& path) {
}
void Browser::ClearRecentDocuments() {
}
void Browser::SetAppUserModelID(const base::string16& name) {
}
bool Browser::RemoveAsDefaultProtocolClient(const std::string& protocol) {
  return false;
}
bool Browser::SetAsDefaultProtocolClient(const std::string& protocol) {
  return false;
}
std::string Browser::GetExecutableFileVersion() const {
  return brightray::GetApplicationVersion();
}
std::string Browser::GetExecutableFileProductName() const {
  return brightray::GetApplicationName();
}
}