Add History.TopHostsVisitsByRank UMA metric.

components/history/core/browser/history_backend.cc

Index: components/history/core/browser/history_backend.cc
diff --git a/components/history/core/browser/history_backend.cc b/components/history/core/browser/history_backend.cc
index d04c038d5bef7fd7c6d97a6bd878e7288a5b94ef..cc0654e9746f536d8fd929d79bed616ea34ec8ff 100644
--- a/components/history/core/browser/history_backend.cc
+++ b/components/history/core/browser/history_backend.cc
@@ -37,6 +37,7 @@
 #include "components/history/core/browser/keyword_search_term.h"
 #include "components/history/core/browser/page_usage_data.h"
 #include "components/history/core/browser/typed_url_syncable_service.h"
+#include "components/history/core/browser/url_utils.h"
 #include "components/history/core/browser/visit_filter.h"
 #include "net/base/registry_controlled_domains/registry_controlled_domain.h"
 #include "sql/error_delegate_util.h"
@@ -89,6 +90,8 @@ const int kMaxRedirectCount = 32;
 // and is deleted.
 const int kExpireDaysThreshold = 90;
 
+const int kMaxTopHostsForMetrics = 50;
+
 // Converts from PageUsageData to MostVisitedURL. |redirects| is a
 // list of redirects for this URL. Empty list means no redirects.
 MostVisitedURL MakeMostVisitedURL(const PageUsageData& page_data,
@@ -416,7 +419,13 @@ TopHostsList HistoryBackend::TopHosts(int num_hosts) const {
   if (!db_)
     return TopHostsList();
 
-  return db_->TopHosts(num_hosts);
+  auto top_hosts = db_->TopHosts(num_hosts);
+
+  host_ranks_.clear();
+  int i = 0;
+  for (const auto& host_count : top_hosts)
+    host_ranks_[host_count.first] = ++i;
+  return top_hosts;
 }
 
 void HistoryBackend::AddPage(const HistoryAddPageArgs& request) {
@@ -725,6 +734,15 @@ void HistoryBackend::CloseAllDatabases() {
   }
 }
 
+void HistoryBackend::RecordTopHostsMetrics(const GURL& url) {
+  auto it = host_ranks_.find(HostForTopHosts(url));
+  int host_rank =
+      it != host_ranks_.end() ? it->second : (kMaxTopHostsForMetrics + 1);
+
+  UMA_HISTOGRAM_ENUMERATION("History.TopHostsVisitsByRank", host_rank,
+                            kMaxTopHostsForMetrics + 2);
+}
+
 std::pair<URLID, VisitID> HistoryBackend::AddPageVisit(
     const GURL& url,
     Time time,
@@ -745,6 +763,11 @@ std::pair<URLID, VisitID> HistoryBackend::AddPageVisit(
        transition_type == ui::PAGE_TRANSITION_KEYWORD_GENERATED))
     typed_increment = 1;
 
+  if (!host_ranks_.empty() && visit_source == SOURCE_BROWSED &&
+      (transition & ui::PAGE_TRANSITION_CHAIN_END)) {
+    RecordTopHostsMetrics(url);
+  }
+
   // See if this URL is already in the DB.
   URLRow url_info(url);
   URLID url_id = db_->GetRowForURL(url, &url_info);