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
#include "atom/browser/browser.h"
#include <atlbase.h>
#include <propkey.h>
#include <windows.h>
#include <shlobj.h>
#include <shobjidl.h>
#include "base/base_paths.h"
#include "base/file_version_info.h"
#include "base/files/file_path.h"
#include "base/memory/scoped_ptr.h"
#include "base/path_service.h"
#include "base/strings/string_util.h"
#include "base/strings/stringprintf.h"
#include "base/strings/utf_string_conversions.h"
#include "base/win/win_util.h"
#include "base/win/registry.h"
#include "base/win/windows_version.h"
#include "atom/common/atom_version.h"
namespace atom {
namespace {
const wchar_t kAppUserModelIDFormat[] = L"electron.app.$1";
BOOL CALLBACK WindowsEnumerationHandler(HWND hwnd, LPARAM param) {