[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..d9696dcb9e5588e44974b5c9ec045e1fa6e1d509 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 +52,25 @@ 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;
 
+  // A tab node id is part of the identifier for the sync tab objects. Tab node
+  // ids are not used for interacting with the model/browser tabs. However, when
+  // when we want to delete a foreign session, we use these values to inform
+  // sync which tabs to delete. We are extracting these tab node ids from
+  // individual session (tab, not header) specifics, but store them here in the
+  // SyncedSession during runtime. We do this because tab node ids may be reused
+  // for different tabs, and tracking which tab id is currently associated with
+  // each tab node id is both difficult and unnecessary. See comments at
+  // SyncedSessionTracker::GetTabImpl for a concrete example of id reuse.
+  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.