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/api/atom_api_app.cc
#include "atom/common/native_mate_converters/image_converter.h"
#include "atom/common/native_mate_converters/net_converter.h"
#include "atom/common/native_mate_converters/value_converter.h"
#include "atom/common/node_includes.h"
#include "atom/common/options_switches.h"
#include "base/command_line.h"
#include "base/environment.h"
#include "base/files/file_path.h"
#include "base/files/file_util.h"
#include "base/path_service.h"
#include "brightray/browser/brightray_paths.h"
#include "chrome/common/chrome_paths.h"
#include "content/public/browser/client_certificate_delegate.h"
#include "content/public/browser/gpu_data_manager.h"
#include "content/public/browser/render_frame_host.h"
#include "content/public/common/content_switches.h"
#include "native_mate/dictionary.h"
#include "native_mate/object_template_builder.h"
#include "net/ssl/ssl_cert_request_info.h"
#include "ui/base/l10n/l10n_util.h"
#include "ui/gfx/image/image.h"
#if defined(OS_WIN)
#include "base/strings/utf_string_conversions.h"
#endif
using atom::Browser;
namespace mate {
#if defined(OS_WIN)
template<>
struct Converter<Browser::UserTask> {
  static bool FromV8(v8::Isolate* isolate, v8::Local<v8::Value> val,