Cleanup: Pass std::string as const reference if possible

sync/test/fake_server/bookmark_entity.h

Index: sync/test/fake_server/bookmark_entity.h
diff --git a/sync/test/fake_server/bookmark_entity.h b/sync/test/fake_server/bookmark_entity.h
index 5f4528d4ac412797d723f53a889cdf31c27068af..c146f86b80c268867e042e3b3631ab3f4e7718a4 100644
--- a/sync/test/fake_server/bookmark_entity.h
+++ b/sync/test/fake_server/bookmark_entity.h
@@ -49,7 +49,7 @@ class BookmarkEntity : public FakeServerEntity {
                  int64 creation_time,
                  int64 last_modified_time);
 
-  void SetParentId(std::string parent_id);
+  void SetParentId(const std::string& parent_id);
 
   // FakeServerEntity implementation.
   std::string GetParentId() const override;