precache: Add geometric resource weight function.

components/precache/core/precache_fetcher.h

Index: components/precache/core/precache_fetcher.h
diff --git a/components/precache/core/precache_fetcher.h b/components/precache/core/precache_fetcher.h
index 87d1c4aa2370d3d11f14df24ce5e79bf532ac0f6..9868d1c015a84d0f26922e4971f162cb5dd8f389 100644
--- a/components/precache/core/precache_fetcher.h
+++ b/components/precache/core/precache_fetcher.h
@@ -20,6 +20,7 @@
 #include "base/single_thread_task_runner.h"
 #include "base/time/time.h"
 #include "components/precache/core/fetcher_pool.h"
+#include "components/precache/core/proto/precache.pb.h"
 #include "components/precache/core/proto/quota.pb.h"
 #include "net/url_request/url_fetcher.h"
 #include "net/url_request/url_fetcher_delegate.h"
@@ -270,6 +271,12 @@ class PrecacheFetcher : public base::SupportsWeakPtr<PrecacheFetcher> {
   DISALLOW_COPY_AND_ASSIGN(PrecacheFetcher);
 };
 
+// Visible for testing.
+double ResourceWeight(
+    PrecacheConfigurationSettings::ResourceWeightFunction function,
+    double resource_weight_ratio,
+    int64_t host_visits);
+
 // Class that fetches a URL, and runs the specified callback when the fetch is
 // complete. This class exists so that a different method can be run in
 // response to different kinds of fetches, e.g. OnConfigFetchComplete when