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 (protocol.empty())
    return false;
  base::FilePath path;
  if (!PathService::Get(base::FILE_EXE, &path)) {
    LOG(ERROR) << "Error getting app exe path";
    return false;
  }
  
  HKEY root = HKEY_CURRENT_USER;
  std::string keyPathStr = "Software\\Classes\\" + protocol;
  std::wstring keyPath = std::wstring(keyPathStr.begin(), keyPathStr.end());
  std::string urlDeclStr = "URL:" + protocol;
  std::wstring urlDecl = std::wstring(urlDeclStr.begin(), urlDeclStr.end());
  
  std::string cmdPathStr = keyPathStr + "\\shell\\open\\command";
  std::wstring cmdPath = std::wstring(cmdPathStr.begin(), cmdPathStr.end());
  
  std::wstring exePath(path.value());
  std::wstring exe = L"\"" + exePath + L"\" \"%1\"";
  
  base::win::RegKey key(root, keyPath.c_str(), KEY_ALL_ACCESS);
  if (FAILED(key.WriteValue(L"URL Protocol", L"")) ||
      FAILED(key.WriteValue(L"", urlDecl.c_str())))
    return false;
  base::win::RegKey commandKey(root, cmdPath.c_str(), KEY_ALL_ACCESS);
  if (FAILED(commandKey.WriteValue(L"", exe.c_str())))
    return false;
  return true;
}
PCWSTR Browser::GetAppUserModelID() {