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

components/sync_sessions/synced_session.h

Index: components/sync_sessions/synced_session.h
diff --git a/components/sync_sessions/synced_session.h b/components/sync_sessions/synced_session.h
index 5e9d7ba48fc902a6b19aed4cdb01e9edd7c99dd4..7c30ec53f1d5653cdab5cff4225827ca84e055af 100644
--- a/components/sync_sessions/synced_session.h
+++ b/components/sync_sessions/synced_session.h
@@ -6,6 +6,7 @@
 #define COMPONENTS_SYNC_SESSIONS_SYNCED_SESSION_H_
 
 #include <map>
+#include <set>
 #include <string>
 
 #include "base/macros.h"
@@ -51,13 +52,19 @@ struct SyncedSession {
   // 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;
 
+  // All of the known to be valid tab node ids associated with this session.
+  // This is not stored in the header, but rather assembled for all tabs, both
+  // parented and orphaned.
+  std::set<int> tab_node_ids;
+
   // 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.