Correct iOS build for latest Xcode beta.

ios/net/empty_nsurlcache.mm

Index: ios/net/empty_nsurlcache.mm
diff --git a/ios/net/empty_nsurlcache.mm b/ios/net/empty_nsurlcache.mm
new file mode 100644
index 0000000000000000000000000000000000000000..dc748189da11970084725c9a46c2165c6a98ed9e
--- /dev/null
+++ b/ios/net/empty_nsurlcache.mm
@@ -0,0 +1,14 @@
+// Copyright 2015 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 "ios/net/empty_nsurlcache.h"
+
+@implementation EmptyNSURLCache
+
+- (nullable NSCachedURLResponse*)cachedResponseForRequest:
+    (nonnull NSURLRequest*)request {
+  return nil;
+}
+
+@end