[Sync] Replacing NULL with nullptr/null throughout sync code.

components/sync_sessions/sync_sessions_metrics.cc

 // Copyright 2016 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.
 
 #include "components/sync_sessions/sync_sessions_metrics.h"
 
 #include <algorithm>
 #include <utility>
 
 #include "base/logging.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/metrics/user_metrics.h"
 #include "base/time/time.h"
 #include "components/sessions/core/session_types.h"
 #include "components/sync_sessions/sessions_sync_manager.h"
 #include "components/sync_sessions/synced_session.h"
 
 namespace sync_sessions {
 
 // static
 void SyncSessionsMetrics::RecordYoungestForeignTabAgeOnNTP(
     SessionsSyncManager* sessions_sync_manager) {
-  if (sessions_sync_manager != NULL) {
+  if (sessions_sync_manager != nullptr) {
     std::vector<const SyncedSession*> foreign_sessions;
     sessions_sync_manager->GetAllForeignSessions(&foreign_sessions);
     base::Time best(MaxTabTimestamp(foreign_sessions));
     base::Time now(base::Time::Now());
     // Don't emit metrics if the foreign tab is timestamped in the future. While
     // the timestamp is set on a different machine, and we may lose some
     // fraction of metrics to clock skew, we don't want the potential to have
     // bad machines with clocks many hours off causing lots of seemingly 0
     // second entries.
     if (base::Time::UnixEpoch() < best && best <= now) {
@@ skipping 19 matching lines @@
     for (const auto& key_value : session->windows) {
       for (const auto& tab : key_value.second->tabs) {
         best = std::max(best, tab->timestamp);
       }
     }
   }
   return best;
 }
 
 }  // namespace sync_sessions