Correct iOS build for latest Xcode beta.

ios/net/empty_nsurlcache.h

Index: ios/net/empty_nsurlcache.h
diff --git a/ios/net/empty_nsurlcache.h b/ios/net/empty_nsurlcache.h
new file mode 100644
index 0000000000000000000000000000000000000000..8547065911e3aa1bd43e1f2163005685f9f62d3e
--- /dev/null
+++ b/ios/net/empty_nsurlcache.h
@@ -0,0 +1,16 @@
+// 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_EMPTY_NSURLCACHE_H_
+#define IOS_NET_EMPTY_NSURLCACHE_H_
+
+#import <Foundation/Foundation.h>
+
+// The RequestTracker can be created on one thread and used on a different one.
+@interface EmptyNSURLCache : NSURLCache
+- (nullable NSCachedURLResponse*)cachedResponseForRequest:
+    (nonnull NSURLRequest*)request;
+@end
+
+#endif  // IOS_NET_EMPTY_NSURLCACHE_H_