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 5ed9c78fdc4b625ac2609ca62f6811fb1ec9a855..722710494effff91e8d81212338f4e4654cb5a0d 100644
--- a/chrome/browser/sync/sessions2/sessions_sync_manager.h
+++ b/chrome/browser/sync/sessions2/sessions_sync_manager.h
@@ -151,6 +151,12 @@ 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
@@ -335,6 +341,10 @@ 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<LocalEventRouter> local_event_router_;
 
   DISALLOW_COPY_AND_ASSIGN(SessionsSyncManager);