Add revalidation_only option to precache config.

components/precache/core/proto/precache.proto

 // Copyright 2013 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.
 
 syntax = "proto2";
 
 package precache;
 
 // Chrome requires this.
 option optimize_for = LITE_RUNTIME;
@@ skipping 87 matching lines @@
   // |total_resources_count| resource URLs are fetched.
   optional uint32 total_resources_count = 7 [default = 999999];
 
   // The minimum visit-adjusted weight for which a resource will be downloaded.
   optional double min_weight = 8 [default = 0];
 
   // Whether to sort resources by weight, descending, before fetching. This
   // affects the fetcher's behavior with respect to max_bytes_total and
   // total_resources_count.
   optional bool global_ranking = 9 [default = false];
+
+  // If true, resource fetches are only made over the network for a given URL if
+  // an existing cache entry exists and has revalidation headers.
+  optional bool revalidation_only = 10 [default = false];
 };