sync: add garbage collection to SessionsSyncManager

chrome/browser/sync/sessions2/sessions_sync_manager.h

Index: chrome/browser/sync/sessions2/sessions_sync_manager.h
diff --git a/chrome/browser/sync/sessions2/sessions_sync_manager.h b/chrome/browser/sync/sessions2/sessions_sync_manager.h
index 8ba12610021c4f463734faf325fddaa626d7883f..366de05a7e64690a0da426e4fb3ff0393e27bdc2 100644
--- a/chrome/browser/sync/sessions2/sessions_sync_manager.h
+++ b/chrome/browser/sync/sessions2/sessions_sync_manager.h
@@ -145,6 +145,12 @@ class SessionsSyncManager : public syncer::SyncableService,
 
   FaviconCache* GetFaviconCache();
 
+  // Triggers garbage collection of stale sessions (as defined by
+  // |stale_session_threshold_days_|). This is called automatically every
+  // time we start up (via AssociateModels) and when new sessions data is
+  // downloaded (sync cycles complete).
+  void DoGarbageCollection();
+
  private:
   // Keep all the links to local tab data in one place. A tab_node_id and tab
   // must be passed at creation. The tab_node_id is not mutable, although
@@ -329,6 +335,10 @@ class SessionsSyncManager : public syncer::SyncableService,
   // client.
   int local_session_header_node_id_;
 
+  // Number of days without activity after which we consider a session to be
+  // stale and a candidate for garbage collection.
+  size_t stale_session_threshold_days_;
+
   scoped_ptr<LocalSessionEventRouter> local_event_router_;
 
   DISALLOW_COPY_AND_ASSIGN(SessionsSyncManager);