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

sync/internal_api/public/base/unique_position.h

Index: sync/internal_api/public/base/unique_position.h
diff --git a/sync/internal_api/public/base/unique_position.h b/sync/internal_api/public/base/unique_position.h
index 70e9b39e09d25ba8e480aff8a769b52cb5c2e56e..f41a51134f4bd187de6c1db9a30dfa84af1ebad7 100644
--- a/sync/internal_api/public/base/unique_position.h
+++ b/sync/internal_api/public/base/unique_position.h
@@ -95,6 +95,8 @@ class SYNC_EXPORT UniquePosition {
   // Returns the suffix.
   std::string GetSuffixForTest() const;
 
+  size_t compressed_size() const { return compressed_.size(); }
+
   // Performs a lossy conversion to an int64_t position.  Positions converted to
   // and from int64_ts using this and the FromInt64 function should maintain
   // their