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
BOUNCE_CRITICAL = 0,
    BOUNCE_INFORMATIONAL = 10,
  };
  int DockBounce(BounceType type);
  void DockCancelBounce(int request_id);
  
  void DockSetBadgeText(const std::string& label);
  std::string DockGetBadgeText();
  
  void DockHide();
  void DockShow();
  
  void DockSetMenu(ui::MenuModel* model);
  
  void DockSetIcon(const gfx::Image& image);
#endif  
#if defined(OS_WIN)
  struct UserTask {
    base::FilePath program;
    base::string16 arguments;
    base::string16 title;
    base::string16 description;
    base::FilePath icon_path;
    int icon_index;
  };