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

sync/syncable/parent_child_index.cc

Index: sync/syncable/parent_child_index.cc
diff --git a/sync/syncable/parent_child_index.cc b/sync/syncable/parent_child_index.cc
index bf6e03fbd2044f55ea01d7b47ea2aa3b1b552c83..18c196b526ad40baf5e15c5e8a00c4d16d8b5638 100644
--- a/sync/syncable/parent_child_index.cc
+++ b/sync/syncable/parent_child_index.cc
@@ -12,6 +12,16 @@
 namespace syncer {
 namespace syncable {
 
+namespace {
+
+template <typename Key, typename Value>
+size_t GetRbTreeMemoryUsgae(size_t num_nodes) {
+  // 4 pointers are stored for value, left, right and parent nodes in most
+  // implementations.
+  return (sizeof(std::pair<Key, Value>) + 4 * sizeof(void*)) * num_nodes;
+}
+}
+
 bool ChildComparator::operator()(const EntryKernel* a,
                                  const EntryKernel* b) const {
   const UniquePosition& a_pos = a->ref(UNIQUE_POSITION);
@@ -193,6 +203,21 @@ const OrderedChildSet* ParentChildIndex::GetSiblings(EntryKernel* e) const {
   return siblings;
 }
 
+size_t ParentChildIndex::MemoryUsage() const {
+  size_t total = model_type_root_ids_.capacity() * sizeof(std::string) +
+                 sizeof(this) + type_root_child_sets_.size() * sizeof(void*);
+  for (const auto& id : model_type_root_ids_)
+    total += id.value().capacity() + 1;
+  total += GetRbTreeMemoryUsgae<Id, void*>(parent_children_map_.size());
+  for (const auto& entry : parent_children_map_)
+    total += entry.first.value().capacity() + 1;
+  for (const auto& child_set : type_root_child_sets_) {
+    if (child_set)
+      total += GetRbTreeMemoryUsgae<void*, void*>(child_set->size());
+  }
+  return total;
+}
+
 /* static */
 bool ParentChildIndex::ShouldUseParentId(const Id& parent_id,
                                          ModelType model_type) {