Sync: Handle empty (implicit) ParentId in GetPredecessorId and GetSuccessorId

sync/syncable/parent_child_index.h

Index: sync/syncable/parent_child_index.h
diff --git a/sync/syncable/parent_child_index.h b/sync/syncable/parent_child_index.h
index 5b51597e2247436ca426cd2bbb4175d9d812d1ec..84c0771403e566642e6065962f463bb0d9cde16a 100644
--- a/sync/syncable/parent_child_index.h
+++ b/sync/syncable/parent_child_index.h
@@ -49,7 +49,14 @@ class SYNC_EXPORT_PRIVATE ParentChildIndex {
 
   // Returns all children of the entry with the given Id.  Returns NULL if the
   // node has no children or the Id does not identify a valid directory node.
-  const OrderedChildSet* GetChildren(const Id& id);
+  const OrderedChildSet* GetChildren(const Id& id) const;
+
+  // Returns all children of the entry.  Returns NULL if the node has no
+  // children.
+  const OrderedChildSet* GetChildren(EntryKernel* e) const;
+
+  // Returns all siblings of the entry.
+  const OrderedChildSet* GetSiblings(EntryKernel* e) const;
 
  private:
   friend class ParentChildIndexTest;