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

sync/syncable/parent_child_index.h

 // Copyright (c) 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.
 
 #ifndef SYNC_SYNCABLE_PARENT_CHILD_INDEX
 #define SYNC_SYNCABLE_PARENT_CHILD_INDEX
 
 #include <map>
 #include <set>
 
@@ skipping 31 matching lines @@
   bool Insert(EntryKernel* e);
 
   // Removes a given child from the index.
   void Remove(EntryKernel* e);
 
   // Returns true if this item is in the index as a child.
   bool Contains(EntryKernel* e) const;
 
   // 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;
   typedef std::map<Id, OrderedChildSet*> ParentChildrenMap;
 
   // Determines entry's model type.
   static ModelType GetModelType(EntryKernel* e);
 
   // Returns parent ID for the entry which is either its PARENT_ID value
   // or derived from its model type.
   const Id& GetParentId(EntryKernel* e) const;
 
   // Returns previously cached model type root ID for the given |model_type|.
   const Id& GetModelTypeRootId(ModelType model_type) const;
 
   // A map of parent IDs to children.
   // Parents with no children are not included in this map.
   ParentChildrenMap parent_children_map_;
 
   // This array tracks model type roots IDs.
   Id model_type_root_ids_[MODEL_TYPE_COUNT];
 
   DISALLOW_COPY_AND_ASSIGN(ParentChildIndex);
 };
 
 }  // namespace syncable
 }  // namespace syncer
 
 #endif  // SYNC_SYNCABLE_PARENT_CHILD_INDEX