Add UMA to log the size of the Previews opt out DB

components/previews/core/previews_opt_out_store_sql.cc

Index: components/previews/core/previews_opt_out_store_sql.cc
diff --git a/components/previews/core/previews_opt_out_store_sql.cc b/components/previews/core/previews_opt_out_store_sql.cc
index d4e6a59eac658be2f87160b297f2300a116749e2..6feb4a16ffa5ed3a4f0d7d8b6cca03d52efae9ee 100644
--- a/components/previews/core/previews_opt_out_store_sql.cc
+++ b/components/previews/core/previews_opt_out_store_sql.cc
@@ -3,20 +3,21 @@
 // found in the LICENSE file.
 
 #include "components/previews/core/previews_opt_out_store_sql.h"
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/files/file_util.h"
 #include "base/location.h"
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/sequenced_task_runner.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "components/previews/core/previews_black_list.h"
 #include "components/previews/core/previews_black_list_item.h"
 #include "components/previews/core/previews_experiments.h"
 #include "sql/connection.h"
 #include "sql/recovery.h"
 #include "sql/statement.h"
 #include "sql/transaction.h"
 
@@ -170,21 +171,22 @@ void LoadBlackListFromDataBase(
     DCHECK_LE(black_list_item_map->size(),
               params::MaxInMemoryHostsInBlackList());
     // Allows the internal logic of PreviewsBlackListItem to determine how to
     // evict entries when there are more than
     // |StoredHistoryLengthForBlackList()| for the host.
     black_list_item->AddPreviewNavigation(
         statement.ColumnBool(2),
         base::Time::FromInternalValue(statement.ColumnInt64(1)));
   }
 
-  // TODO(ryansturm): Add UMA to log |count|. crbug.com/656739
+  UMA_HISTOGRAM_COUNTS_10000("Previews.OptOut.DBRowCount", count);
+
   if (count > kMaxRowsInDB) {
     // Delete the oldest entries if there are more than |kMaxEntriesInDB|.
     // DELETE ... LIMIT -1 OFFSET x means delete all but the first x entries.
     const char kSqlDeleteByDBSize[] = "DELETE FROM " PREVIEWS_TABLE_NAME
                                       " WHERE ROWID IN"
                                       " (SELECT ROWID from " PREVIEWS_TABLE_NAME
                                       " ORDER BY time DESC"
                                       " LIMIT -1 OFFSET ?)";
 
     sql::Statement statement_delete(