CrNet: add pauseable NSURLProtocol and switch to using it

ios/net/crn_http_protocol_handler_proxy.h

 // Copyright 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.
 
 #ifndef IOS_NET_CRN_HTTP_PROTOCOL_HANDLER_PROXY_H_
 #define IOS_NET_CRN_HTTP_PROTOCOL_HANDLER_PROXY_H_
 
 #import <Foundation/Foundation.h>
 
 #import "ios/net/clients/crn_network_client_protocol.h"
 
 // HttpProtocolHandlerProxy is responsible for managing access to the
 // NSURLProtocol and its client.
 @protocol CRNHTTPProtocolHandlerProxy<CRNNetworkClientProtocol>
 
+// All of the methods defined below must be called on the client thread. Methods
+// defined by |CRNNetworkClientProtocol| can be called on any thread.
+
 // Invalidates any reference to the protocol handler: the handler will never
 // be called after this.
-// Called from the client thread.
 - (void)invalidate;
 
+// Pauses notifications from this protocol handler.
+- (void)pause;
+
+// Resumes notifications from this protocol handler.
+- (void)resume;
+
 @end
 
 #endif  // IOS_NET_CRN_HTTP_PROTOCOL_HANDLER_PROXY_H_