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 72bcc066029b9eb3cb9a9b8ad955b352b484ed52..dd80621a3b66ac57cf1fa8b1d19ecc0a77fac899 100644
--- a/components/precache/core/proto/precache.proto
+++ b/components/precache/core/proto/precache.proto
@@ -109,4 +109,18 @@ message PrecacheConfigurationSettings {
   // 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];
+
+  // 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];
 };