[Sync] Introduce SyncedSessionWindow type.

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"
@@ skipping 33 matching lines @@
   // While Sessions are ordered by recency, windows and tabs are not. Because
   // the timestamp of sessions are updated when windows/tabs are removed, we
   // only need to search until all the remaining sessions are older than the
   // most recent tab we've found so far.
   base::Time best(base::Time::UnixEpoch());
   for (const SyncedSession* session : sessions) {
     if (session->modified_time < best) {
       break;
     }
     for (const auto& key_value : session->windows) {
-      for (const auto& tab : key_value.second->tabs) {
+      for (const auto& tab : key_value.second->wrapped_window.tabs) {
         best = std::max(best, tab->timestamp);
       }
     }
   }
   return best;
 }
 
 }  // namespace sync_sessions