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.h
void Exit(int code);
  
  void Shutdown();
  
  void Focus();
  
  std::string GetVersion() const;
  
  void SetVersion(const std::string& version);
  
  std::string GetName() const;
  
  void SetName(const std::string& name);
  
  void AddRecentDocument(const base::FilePath& path);
  
  void ClearRecentDocuments();
  
  void SetAppUserModelID(const base::string16& name);
  
  bool RemoveAsDefaultProtocolClient(const std::string& protocol);
  
  bool SetAsDefaultProtocolClient(const std::string& protocol);
#if defined(OS_MACOSX)
  
  void Hide();
  
  void Show();
  
  enum BounceType {