Enable Null Syncable ID which is different than Root ID.

sync/syncable/mutable_entry.cc

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "sync/syncable/mutable_entry.h"
 
 #include "base/memory/scoped_ptr.h"
 #include "sync/internal_api/public/base/unique_position.h"
 #include "sync/syncable/directory.h"
 #include "sync/syncable/scoped_kernel_lock.h"
@@ skipping 209 matching lines @@
     DCHECK(value.IsValid());
     ScopedKernelLock lock(dir());
     ScopedParentChildIndexUpdater updater(
         lock, kernel_, &dir()->kernel_->parent_child_index);
     kernel_->put(UNIQUE_POSITION, value);
     kernel_->mark_dirty(&dir()->kernel_->dirty_metahandles);
   }
 }
 
 bool MutableEntry::PutPredecessor(const Id& predecessor_id) {
-  MutableEntry predecessor(write_transaction(), GET_BY_ID, predecessor_id);
-  if (!predecessor.good())
-    return false;
-  dir()->PutPredecessor(kernel_, predecessor.kernel_);
+  if (predecessor_id.IsNull()) {
+    dir()->PutPredecessor(kernel_, NULL);
+  } else {
+    MutableEntry predecessor(write_transaction(), GET_BY_ID, predecessor_id);
+    if (!predecessor.good())
+      return false;
+    dir()->PutPredecessor(kernel_, predecessor.kernel_);
+  }
   return true;
 }
 
 void MutableEntry::PutAttachmentMetadata(
     const sync_pb::AttachmentMetadata& attachment_metadata) {
   DCHECK(kernel_);
   write_transaction()->TrackChangesTo(kernel_);
   if (kernel_->ref(ATTACHMENT_METADATA).SerializeAsString() !=
       attachment_metadata.SerializeAsString()) {
     dir()->UpdateAttachmentIndex(GetMetahandle(),
@@ skipping 27 matching lines @@
   DCHECK_NE(static_cast<MutableEntry*>(NULL), e);
   DCHECK(!e->IsRoot()) << "We shouldn't mark a permanent object for syncing.";
   if (!(e->PutIsUnsynced(true)))
     return false;
   e->PutSyncing(false);
   return true;
 }
 
 }  // namespace syncable
 }  // namespace syncer