Add prefetch UMA metrics.

components/precache/core/precache_fetcher.h

 // 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.
 
 #ifndef COMPONENTS_PRECACHE_CORE_PRECACHE_FETCHER_H_
 #define COMPONENTS_PRECACHE_CORE_PRECACHE_FETCHER_H_
 
 #include <list>
 #include <string>
 #include <vector>
@@ skipping 103 matching lines @@
   // The request context used when fetching URLs.
   scoped_refptr<net::URLRequestContextGetter> request_context_;
 
   // The custom URL prefix to use when fetching manifests. If not provided, the
   // default flag-specified prefix will be used.
   const std::string manifest_url_prefix_;
 
   // 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_;
 
   DISALLOW_COPY_AND_ASSIGN(PrecacheFetcher);
 };
 
 }  // namespace precache
 
 #endif  // COMPONENTS_PRECACHE_CORE_PRECACHE_FETCHER_H_