[WIP] refactor: (Part I) make the ownership of URLRequestContextGetter more clear (#13956)

* refactor: desttroy URLRequestContextGetter on IO thread

* Accepts a factory class that can customize the creation of URLRequestContext
* Use a separate request context for media which is derived from the default
* Notify URLRequestContextGetter observers and cleanup on IO thread
* Move most of brightray net/ classes into atom net/

* refactor: remove refs to URLRequestContextGetter on shutdown

* refactor: remove brigtray switches.{cc|h}

* refactor: remove brightray network_delegate.{cc|h}

* refactor: make AtomURLRequestJobFactory the top level factory.

* Allows to use the default handler from content/ for http{s}, ws{s} schemes.
* Removes the storage of job factory in URLRequestContextGetter.

atom/browser/net/http_protocol_handler.cc

@@ -1,22 +0,0 @@
-// Copyright (c) 2015 GitHub, Inc.
-// Use of this source code is governed by the MIT license that can be
-// found in the LICENSE file.
-
-#include "atom/browser/net/http_protocol_handler.h"
-
-#include "net/url_request/url_request_http_job.h"
-
-namespace atom {
-
-HttpProtocolHandler::HttpProtocolHandler(const std::string& scheme)
-    : scheme_(scheme) {}
-
-HttpProtocolHandler::~HttpProtocolHandler() {}
-
-net::URLRequestJob* HttpProtocolHandler::MaybeCreateJob(
-    net::URLRequest* request,
-    net::NetworkDelegate* network_delegate) const {
-  return net::URLRequestHttpJob::Factory(request, network_delegate, scheme_);
-}
-
-}  // namespace atom