Add revalidation_only option to precache config.

components/precache/core/proto/precache.proto

Index: components/precache/core/proto/precache.proto
diff --git a/components/precache/core/proto/precache.proto b/components/precache/core/proto/precache.proto
index f5124bcc2d3bcb8f288cd9fb93ec36895edd90e5..72bcc066029b9eb3cb9a9b8ad955b352b484ed52 100644
--- a/components/precache/core/proto/precache.proto
+++ b/components/precache/core/proto/precache.proto
@@ -105,4 +105,8 @@ message PrecacheConfigurationSettings {
   // 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];
 };