[Sync] Handle reassociation of tabs where the old tab is already mapped.

components/sync_sessions/synced_session_tracker.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_TRACKER_H_
 #define COMPONENTS_SYNC_SESSIONS_SYNCED_SESSION_TRACKER_H_
 
 #include <stddef.h>
 
 #include <map>
@@ skipping 156 matching lines @@
   bool GetTabNodeFromLocalTabId(SessionID::id_type tab_id, int* tab_node_id);
 
   // Returns whether |tab_node_id| refers to a valid tab node that is associated
   // with a tab.
   bool IsLocalTabNodeAssociated(int tab_node_id);
 
   // Reassociates the tab denoted by |tab_node_id| with a new tab id, preserving
   // any previous SessionTab object the node was associated with. This is useful
   // on restart when sync needs to reassociate tabs from a previous session with
   // newly restored tabs (and can be used in conjunction with PutTabInWindow).
+  // If |new_tab_id| is already associated with a tab object, that tab will be
+  // overwritten. Reassociating a tab with a node it is already mapped to will
+  // have no effect.
   void ReassociateLocalTab(int tab_node_id, SessionID::id_type new_tab_id);
 
   // **** Methods for querying/manipulating overall state ****.
 
   // Free the memory for all dynamically allocated objects and clear the
   // tracking structures.
   void Clear();
 
   bool Empty() const {
     return synced_tab_map_.empty() && synced_session_map_.empty();
@@ skipping 61 matching lines @@
 
   // Pool of used/available sync nodes associated with local tabs.
   TabNodePool local_tab_pool_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncedSessionTracker);
 };
 
 }  // namespace sync_sessions
 
 #endif  // COMPONENTS_SYNC_SESSIONS_SYNCED_SESSION_TRACKER_H_