precache: Add geometric resource weight function.

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];
+
+  // The function to use to combine a resource's weight_ratio with its
+  // referring manifest's host_visits count to produce a final score.
+  enum ResourceWeightFunction {
+    // Models the expected number of requests for the resource in the next 30
+    // days, given that weight_ratio is a probability that a visit to the host
+    // will request a resource, and host_visits is an estimate of the number of
+    // visits to the host in the next 30 days.
+    FUNCTION_NAIVE = 0;
+    // Models the probability of at least one request, given the same.
+    FUNCTION_GEOMETRIC = 1;
+  };
+  optional ResourceWeightFunction resource_weight_function = 11
+      [default = FUNCTION_NAIVE];
 };