[Sync] Avoid unregistering object IDs on shutdown

chrome/browser/sync/glue/sync_backend_host.h

Index: chrome/browser/sync/glue/sync_backend_host.h
diff --git a/chrome/browser/sync/glue/sync_backend_host.h b/chrome/browser/sync/glue/sync_backend_host.h
index bafe1402f07628a69c6ba5e6ba88ab556163d912..dd703b45417c6e40178ab6718bd913a1ce66da6f 100644
--- a/chrome/browser/sync/glue/sync_backend_host.h
+++ b/chrome/browser/sync/glue/sync_backend_host.h
@@ -179,8 +179,8 @@ class SyncBackendHost : public BackendDataTypeConfigurer {
   // Called on |frontend_loop| to update SyncCredentials.
   void UpdateCredentials(const syncer::SyncCredentials& credentials);
 
-  // Registers the underlying frontend for the given IDs to the
-  // underlying notifier.
+  // Registers the underlying frontend for the given IDs to the underlying
+  // notifier.  This lasts until StopSyncingForShutdown() is called.
   void UpdateRegisteredInvalidationIds(const syncer::ObjectIdSet& ids);
 
   // This starts the SyncerThread running a Syncer object to communicate with
@@ -211,9 +211,10 @@ class SyncBackendHost : public BackendDataTypeConfigurer {
   bool SetDecryptionPassphrase(const std::string& passphrase)
       WARN_UNUSED_RESULT;
 
-  // Called on |frontend_loop_| to kick off shutdown procedure. After this,
-  // no further sync activity will occur with the sync server and no further
+  // Called on |frontend_loop_| to kick off shutdown procedure. After this, no
+  // further sync activity will occur with the sync server and no further
   // change applications will occur from changes already downloaded.
+  // Furthermore, no notifications will be sent to any invalidation handler.
   virtual void StopSyncingForShutdown();
 
   // Called on |frontend_loop_| to kick off shutdown.