Enable Null Syncable ID which is different than Root ID.

sync/engine/commit_util.cc

Index: sync/engine/commit_util.cc
diff --git a/sync/engine/commit_util.cc b/sync/engine/commit_util.cc
index c913edd11108c99e159fee7a7ad6536356facf1d..a40870320f003da48e9ce9096fc1edc628daa910 100644
--- a/sync/engine/commit_util.cc
+++ b/sync/engine/commit_util.cc
@@ -150,11 +150,11 @@ void BuildCommitItem(
   // TODO(nick): With the server keeping track of the primary sync parent,
   // it should not be necessary to provide the old_parent_id: the version
   // number should suffice.
-  if (new_parent_id != meta_entry.GetServerParentId() &&
+  Id server_parent_id = meta_entry.GetServerParentId();
+  if (new_parent_id != server_parent_id && !server_parent_id.IsNull() &&

[pavely, 2014/12/22 21:38:09]

Could you give examples when server_parent_id will be empty? It seems like there
is change of behavior that might affect server.

[stanisc, 2014/12/22 21:56:01]

Currently parent IDs are never nulls (empty) so there shouldn't be any change in
the behavior, at least not until the next change that will really introduce
empty parent IDs.
I made this change in anticipation of the future change and to avoid changing a
lot of tests that don't bother setting parent IDs explicitly. With the previous
implementation unset parent IDs would all be root IDs (with "r" as their
content). Now that uninitialized IDs end up being empty strings
SyncableIdToProto fails to process an empty string based ID. 
I had two choices. Either adding an extra check here or modify SyncableIdToProto
to handle a null ID the same way it handled a root ID.

[pavely, 2014/12/22 22:00:48]

sgtm

       0 != meta_entry.GetBaseVersion() &&
       syncable::CHANGES_VERSION != meta_entry.GetBaseVersion()) {
-    sync_entry->set_old_parent_id(
-        SyncableIdToProto(meta_entry.GetServerParentId()));
+    sync_entry->set_old_parent_id(SyncableIdToProto(server_parent_id));
   }
 
   int64 version = meta_entry.GetBaseVersion();
@@ -184,7 +184,7 @@ void BuildCommitItem(
       // for legacy reasons.  See comments in sync.proto for more information.
       const Id& prev_id = meta_entry.GetPredecessorId();
       string prev_id_string =
-          prev_id.IsRoot() ? string() : prev_id.GetServerId();
+          prev_id.IsNull() ? string() : prev_id.GetServerId();
       sync_entry->set_insert_after_item_id(prev_id_string);
       sync_entry->set_position_in_parent(
           meta_entry.GetUniquePosition().ToInt64());