Add History.TopHostsVisitsByRank UMA metric.

components/history/core/browser/history_backend.h

Index: components/history/core/browser/history_backend.h
diff --git a/components/history/core/browser/history_backend.h b/components/history/core/browser/history_backend.h
index 76d4b50b0b0c64248453f8b46bd3a76228dcc150..14332bded3cd5566cfb4f9f4938145948a8d793a 100644
--- a/components/history/core/browser/history_backend.h
+++ b/components/history/core/browser/history_backend.h
@@ -10,6 +10,7 @@
 #include <utility>
 #include <vector>
 
+#include "base/containers/hash_tables.h"
 #include "base/containers/mru_cache.h"
 #include "base/files/file_path.h"
 #include "base/gtest_prod_util.h"
@@ -201,6 +202,9 @@ class HistoryBackend : public base::RefCountedThreadSafe<HistoryBackend>,
   // Computes the |num_hosts| most-visited hostnames in the past 30 days. See
   // history_service.h for details. Returns an empty list if db_ is not
   // initialized.
+  //
+  // As a side effect, caches the list of top hosts for the purposes of
+  // generating internal metrics.
   TopHostsList TopHosts(int num_hosts) const;
 
   // Navigation ----------------------------------------------------------------
@@ -518,6 +522,7 @@ class HistoryBackend : public base::RefCountedThreadSafe<HistoryBackend>,
   FRIEND_TEST_ALL_PREFIXES(HistoryBackendTest, TopHosts_OnlyLast30Days);
   FRIEND_TEST_ALL_PREFIXES(HistoryBackendTest, TopHosts_MaxNumHosts);
   FRIEND_TEST_ALL_PREFIXES(HistoryBackendTest, TopHosts_IgnoreUnusualURLs);
+  FRIEND_TEST_ALL_PREFIXES(HistoryBackendTest, RecordTopHostsMetrics);
   FRIEND_TEST_ALL_PREFIXES(HistoryBackendTest, UpdateVisitDuration);
   FRIEND_TEST_ALL_PREFIXES(HistoryBackendTest, ExpireHistoryForTimes);
   FRIEND_TEST_ALL_PREFIXES(HistoryBackendTest, DeleteFTSIndexDatabases);
@@ -727,6 +732,8 @@ class HistoryBackend : public base::RefCountedThreadSafe<HistoryBackend>,
                          const URLRows& rows,
                          const std::set<GURL>& favicon_urls) override;
 
+  void RecordTopHostsMetrics(const GURL& url);
+
   // Deleting all history ------------------------------------------------------
 
   // Deletes all history. This is a special case of deleting that is separated
@@ -832,6 +839,9 @@ class HistoryBackend : public base::RefCountedThreadSafe<HistoryBackend>,
   // Listens for the system being under memory pressure.
   scoped_ptr<base::MemoryPressureListener> memory_pressure_listener_;
 
+  // Map from host to 1-based index in the TopHosts list.
+  mutable base::hash_map<std::string, int> host_ranks_;
+
   // List of observers
   base::ObserverList<HistoryBackendObserver> observers_;