[Cronet] Add BUILD.gn for legacy ios/CrNet project.

ios/crnet/crnet_consumer/main.mm

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