Add some docs to URLRequestContextBuilder.

net/url_request/url_request_context_builder.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/url_request/url_request_context_builder.h"
 
 #include <string>
 #include <utility>
 #include <vector>
 
@@ skipping 116 matching lines @@
 
   bool OnCanAccessFile(const URLRequest& request,
                        const base::FilePath& original_path,
                        const base::FilePath& absolute_path) const override {
     return true;
   }
 
   DISALLOW_COPY_AND_ASSIGN(BasicNetworkDelegate);
 };
 
-// Define a context class that can self-manage the ownership of its components
-// via a UrlRequestContextStorage object. Since it cancels requests in its
-// destructor, it's not safe to subclass this.
+// A URLRequestContext subclass that owns most of its components
+// via a UrlRequestContextStorage object. When URLRequestContextBuilder::Build()
+// is called, ownership of all URLRequestContext components is passed to the
+// ContainerURLRequestContext. Since this cancels requests in its destructor,
+// it's not safe to subclass this.
 class ContainerURLRequestContext final : public URLRequestContext {
  public:
   explicit ContainerURLRequestContext(
       const scoped_refptr<base::SingleThreadTaskRunner>& file_task_runner)
       : file_task_runner_(file_task_runner), storage_(this) {}
 
   ~ContainerURLRequestContext() override {
     // Shut down the ProxyService, as it may have pending URLRequests using this
     // context. Since this cancels requests, it's not safe to subclass this, as
     // some parts of the URLRequestContext may then be torn down before this
@@ skipping 355 matching lines @@
     std::unique_ptr<ProxyConfigService> proxy_config_service,
     URLRequestContext* url_request_context,
     HostResolver* host_resolver,
     NetworkDelegate* network_delegate,
     NetLog* net_log) {
   return ProxyService::CreateUsingSystemProxyResolver(
       std::move(proxy_config_service), net_log);
 }
 
 }  // namespace net