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

ios/crnet/crnet_consumer/crnet_consumer_view_controller.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 "crnet_consumer_view_controller.h"
 
-#import "CrNet.h"
+#import "ios/crnet/CrNet.h"
 
 @implementation CrNetConsumerViewController {
   UIWebView* _webView;
 }
 
 - (void)viewDidLoad {
   self.view.backgroundColor = [UIColor whiteColor];
 
   UIButton* button = [UIButton buttonWithType:UIButtonTypeSystem];
   [button setTitle:@"chromium.org" forState:UIControlStateNormal];
@@ skipping 19 matching lines @@
 - (BOOL)prefersStatusBarHidden {
   return YES;
 }
 
 - (void)loadChromium {
   [_webView loadRequest:[NSURLRequest requestWithURL:
       [NSURL URLWithString:@"https://www.chromium.org"]]];
 }
 
 @end