[Sync] Fix namespaces for the sync_sessions component.

components/sync_sessions/sync_sessions_metrics.h

 // 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.
 
 #ifndef COMPONENTS_SYNC_SESSIONS_SYNC_SESSIONS_METRICS_H_
 #define COMPONENTS_SYNC_SESSIONS_SYNC_SESSIONS_METRICS_H_
 
 #include <vector>
 
 namespace base {
 class Time;
 }  // namespace base
 
-namespace browser_sync {
+namespace sync_sessions {
+
 class SessionsSyncManager;
-}  // namespace browser_sync
-
-namespace sync_driver {
 struct SyncedSession;
-}  // namespace sync_driver
-
-namespace sync_sessions {
 
 class SyncSessionsMetrics {
  public:
   // Records via an UMA histogram the age of the youngest foreign tab the given
   // manager is aware of. No attempt is made to aquire a more recent version of
   // world. If anything goes wrong, such as the manager being null, no foreign
   // session present, or a foreign tab with a timestamp in the future then no
   // metric is emitted.
   static void RecordYoungestForeignTabAgeOnNTP(
-      browser_sync::SessionsSyncManager* sessions_sync_manager);
+      SessionsSyncManager* sessions_sync_manager);
 
  private:
   friend class SyncSessionsMetricsTest;
 
   // Returns the highest timestamp for any tab in the given sessions. Note that
   // sessions should be sorted by timestamp, and session timestamps should
   // always be greater or equal to children window/tab timestamps. No navigation
   // timestamps are checked.
   static base::Time MaxTabTimestamp(
-      const std::vector<const sync_driver::SyncedSession*>& sessions);
+      const std::vector<const SyncedSession*>& sessions);
 };
 
 }  // namespace sync_sessions
 
 #endif  // COMPONENTS_SYNC_SESSIONS_SYNC_SESSIONS_METRICS_H_