Add prefetch UMA metrics.

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 de368419707d7bc55500b11ff15b13c5d7ca134e..67a85c714d6b0311a5eb29e8e296b2686bf2c5f5 100644
--- a/components/precache/core/precache_fetcher.h
+++ b/components/precache/core/precache_fetcher.h
@@ -121,8 +121,13 @@ class PrecacheFetcher {
   // Non-owning pointer. Should not be NULL.
   PrecacheDelegate* precache_delegate_;
 
+  // Tally of the total number of bytes received from URL fetches, including
+  // config, manifests, and resources.
+  int total_response_bytes_;

[bengr, 2015/06/30 19:26:19]

Are we concerned about overflow?

[twifkak, 2015/06/30 20:23:50]

An int should be able to represent up to ~2G, so I'm not concerned. If it does
occur, it should show up in the histogram as a large count in bucket 0 and a
histogram which does not appear to decrease to zero count in the upper buckets
-- i.e. aliasing. But please correct me if my analysis is wrong.

+
   scoped_ptr<Fetcher> fetcher_;
 
+  int num_manifest_urls_to_fetch_;
   std::list<GURL> manifest_urls_to_fetch_;
   std::list<GURL> resource_urls_to_fetch_;