Sync: Change Local IDs to GUID based to avoid ID collision

sync/syncable/directory.h

Index: sync/syncable/directory.h
diff --git a/sync/syncable/directory.h b/sync/syncable/directory.h
index 4f45cdfe18bf0e0e53e9d1df3b2b27283833e52c..2e69de402d30cc2ff590ad5d60139656c6cc131d 100644
--- a/sync/syncable/directory.h
+++ b/sync/syncable/directory.h
@@ -110,8 +110,6 @@ class SYNC_EXPORT Directory {
     // The store birthday we were given by the server. Contents are opaque to
     // the client.
     std::string store_birthday;
-    // The next local ID that has not been used with this cache-GUID.
-    int64 next_id;
     // The serialized bag of chips we were given by the server. Contents are
     // opaque to the client. This is the serialization of a message of type
     // ChipBag defined in sync.proto. It can contains NULL characters.