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

sync/syncable/directory.h

Index: sync/syncable/directory.h
diff --git a/sync/syncable/directory.h b/sync/syncable/directory.h
index bdbfe95c6fa3f2b6504f5637ca9721bf0581b19b..8a98331fa7971f69dc2457e0fe44f03ddea9dbe8 100644
--- a/sync/syncable/directory.h
+++ b/sync/syncable/directory.h
@@ -30,6 +30,12 @@
 #include "sync/syncable/parent_child_index.h"
 #include "sync/syncable/syncable_delete_journal.h"
 
+namespace base {
+namespace trace_event {
+class ProcessMemoryDump;
+}
+}
+
 namespace syncer {
 
 class Cryptographer;
@@ -283,6 +289,9 @@ class SYNC_EXPORT Directory {
   // Gets the total number of entries in the directory.
   size_t GetEntriesCount() const;
 
+  // Adds memory statistics to |pmd| for chrome://tracing.
+  void OnMemoryDump(base::trace_event::ProcessMemoryDump* pmd);
+
   // Gets/Increments transaction version of a model type. Must be called when
   // holding kernel mutex.
   int64_t GetTransactionVersion(ModelType type) const;