[Sync] Replacing NULL with nullptr in code and null in comments for components/sync/

components/sync/syncable/parent_child_index.h

Index: components/sync/syncable/parent_child_index.h
diff --git a/components/sync/syncable/parent_child_index.h b/components/sync/syncable/parent_child_index.h
index 47a1ee50c0dc0d371abc6efe8bbc1ee261260ba9..c354c3b6a9282ab39d70570998690a8198d11089 100644
--- a/components/sync/syncable/parent_child_index.h
+++ b/components/sync/syncable/parent_child_index.h
@@ -49,11 +49,11 @@ class ParentChildIndex {
   // 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
+  // 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;
 
-  // Returns all children of the entry.  Returns NULL if the node has no
+  // Returns all children of the entry.  Returns null if the node has no
   // children.
   const OrderedChildSet* GetChildren(EntryKernel* e) const;