[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.

[Nicolas Zea, 2016/04/12 20:38:03]

Why mention "known to be valid"?

Also "stored in the header" makes it sound like this data somehow gets taken and
pushed into the header node. Similarly, "assembled for all tabs" makes it sound
like the tabs are themselves using this data (rather than this data coming from
the tab nodes). Consider rephrasing?

[skym, 2016/04/12 22:12:14]

Yeah, I have no idea what I was thinking when I typed "known to be valid",
because I'm filtering out invalid tabs in LookupTabNodeIds(...).

Done.

[Nicolas Zea, 2016/04/14 18:17:25]

Much clearer, thanks!

+  // 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.