Add invalidation to url_response_disk_cache.

services/url_response_disk_cache/url_response_disk_cache_entry.mojom

 // 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.
 
 // These mojom structs are declared only to get generated serialization code
 // (ie. they are not used for ipc).
 module mojo;
 
 import "mojo/public/interfaces/network/url_response.mojom";
 
 struct CacheKey {
   string request_origin;
   string url;
   int64 timestamp;
 };
 
 // One entry in the service cache.
 struct CacheEntry {
   URLResponse response;
   string entry_directory;
   string response_body_path;
+  int64 last_invalidation;
 };