[WIP][tracing] Add memory dump provider for sync Directory

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 540234c62b29e81b16557739a626dad8a7a06e14..ea501707fce4b85b683eeb1d777ea49a424d7b6e 100644
--- a/sync/syncable/parent_child_index.h
+++ b/sync/syncable/parent_child_index.h
@@ -60,6 +60,9 @@ class SYNC_EXPORT ParentChildIndex {
   // Returns all siblings of the entry.
   const OrderedChildSet* GetSiblings(EntryKernel* e) const;
 
+  // Returns the memory usage of the index.
+  size_t MemoryUsage() const;
+
  private:
   friend class ParentChildIndexTest;