[Sync] Introduce SyncedSessionWindow type.

components/sync_sessions/sync_sessions_metrics.cc

Index: components/sync_sessions/sync_sessions_metrics.cc
diff --git a/components/sync_sessions/sync_sessions_metrics.cc b/components/sync_sessions/sync_sessions_metrics.cc
index 1242b095e3289cd4ac8df9bbb6be82e7602628e4..1d6ca99925fb491d20b9bf43aac1f65aa7af3f9f 100644
--- a/components/sync_sessions/sync_sessions_metrics.cc
+++ b/components/sync_sessions/sync_sessions_metrics.cc
@@ -51,7 +51,7 @@ base::Time SyncSessionsMetrics::MaxTabTimestamp(
       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);
       }
     }