sync: remove use of protobuf extensions in protocol to reduce static init overhead.

chrome/browser/sync/glue/session_change_processor.cc

Index: chrome/browser/sync/glue/session_change_processor.cc
diff --git a/chrome/browser/sync/glue/session_change_processor.cc b/chrome/browser/sync/glue/session_change_processor.cc
index 8b51ed98694f8878745a042360cf697d44083e87..00b99794983a7fbbc527a20b813c028c353a3cc2 100644
--- a/chrome/browser/sync/glue/session_change_processor.cc
+++ b/chrome/browser/sync/glue/session_change_processor.cc
@@ -264,8 +264,7 @@ void SessionChangeProcessor::ApplyChangesFromSyncModel(
       // and just ensure we've disassociated.
       DCHECK_EQ(syncable::GetModelTypeFromSpecifics(it->specifics),
                 syncable::SESSIONS);
-      const sync_pb::SessionSpecifics& specifics =
-          it->specifics.GetExtension(sync_pb::session);
+      const sync_pb::SessionSpecifics& specifics = it->specifics.session();
       session_model_associator_->DisassociateForeignSession(
           specifics.session_tag());
       continue;