precache: Add geometric resource weight function.

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..f9de162a9ac88fd88d8fc63501b1fdbff6c7bd6a 100644
--- a/components/precache/core/proto/precache.proto
+++ b/components/precache/core/proto/precache.proto
@@ -105,4 +105,18 @@ message PrecacheConfigurationSettings {
   // 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];
 };