[Sync] Moved tab_node_id tracking to session object and improved foreign session garbage collection.

components/sync_sessions/synced_session.h

 // Copyright 2012 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_SYNCED_SESSION_H_
 #define COMPONENTS_SYNC_SESSIONS_SYNCED_SESSION_H_
 
 #include <map>
 #include <string>
 
@@ skipping 33 matching lines @@
   ~SyncedSession();
 
   // Unique tag for each session.
   std::string session_tag;
   // User-visible name
   std::string session_name;
 
   // Type of device this session is from.
   DeviceType device_type;
 
-  // Last time this session was modified remotely.
+  // Last time this session was modified remotely. This is the max of the header
+  // and all children tab mtimes.
   base::Time modified_time;
 
   // Map of windows that make up this session. Windowws are owned by the session
   // itself and free'd on destruction.
   SyncedWindowMap windows;
 
   // Converts the DeviceType enum value to a string. This is used
   // in the NTP handler for foreign sessions for matching session
   // types to an icon style.
   std::string DeviceTypeAsString() const {
@@ skipping 21 matching lines @@
   // does not create SessionTab protobufs.
   sync_pb::SessionHeader ToSessionHeader() const;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(SyncedSession);
 };
 
 }  // namespace sync_driver
 
 #endif  // COMPONENTS_SYNC_SESSIONS_SYNCED_SESSION_H_