[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.

brightray/browser/net/url_request_context_getter_factory.h

@@ -0,0 +1,29 @@
+// Copyright (c) 2018 GitHub, Inc.
+// Use of this source code is governed by the MIT license that can be
+// found in the LICENSE file.
+
+#ifndef BRIGHTRAY_BROWSER_NET_URL_REQUEST_CONTEXT_GETTER_FACTORY_H_
+#define BRIGHTRAY_BROWSER_NET_URL_REQUEST_CONTEXT_GETTER_FACTORY_H_
+
+#include "base/macros.h"
+
+namespace net {
+class URLRequestContext;
+}  // namespace net
+
+namespace brightray {
+
+class URLRequestContextGetterFactory {
+ public:
+  URLRequestContextGetterFactory() {}
+  virtual ~URLRequestContextGetterFactory() {}
+
+  virtual net::URLRequestContext* Create() = 0;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(URLRequestContextGetterFactory);
+};
+
+}  // namespace brightray
+
+#endif  // BRIGHTRAY_BROWSER_NET_URL_REQUEST_CONTEXT_GETTER_FACTORY_H_