CrNet: add pauseable NSURLProtocol and switch to using it

ios/crnet/crnet_consumer/main.m

-// Copyright 2014 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.
-
-#import <Foundation/Foundation.h>
-#import <UIKit/UIKit.h>
-
-#import "crnet_consumer_app_delegate.h"
-
-int main(int argc, char *argv[]) {
-  @autoreleasepool {
-    return UIApplicationMain(
-        argc, argv, nil, NSStringFromClass([CrNetConsumerAppDelegate class]));
-  }
-}