Sync MDP: reimplement value conversions ontop of visitors

components/sync/BUILD.gn

Index: components/sync/BUILD.gn
diff --git a/components/sync/BUILD.gn b/components/sync/BUILD.gn
index d5134afa88fc5b98ead83bedb429ea68d563f706..c09a70b7b0bba4eb1a0bc1492e25a5f17e299bed 100644
--- a/components/sync/BUILD.gn
+++ b/components/sync/BUILD.gn
@@ -472,6 +472,8 @@ static_library("sync") {
     "js/sync_js_controller.h",
     "protocol/proto_enum_conversions.cc",
     "protocol/proto_enum_conversions.h",
+    "protocol/proto_old_value_conversions.cc",
+    "protocol/proto_old_value_conversions.h",
     "protocol/proto_value_conversions.cc",
     "protocol/proto_value_conversions.h",
     "protocol/sync_protocol_error.cc",
@@ -882,7 +884,10 @@ source_set("unit_tests") {
     "engine_impl/worker_entity_tracker_unittest.cc",
     "js/js_event_details_unittest.cc",
     "js/sync_js_controller_unittest.cc",
+    "protocol/proto_compare_value_conversions_unittest.cc",
     "protocol/proto_enum_conversions_unittest.cc",
+    "protocol/proto_unittest_serialized_data.cc",
+    "protocol/proto_unittest_serialized_data.h",
     "protocol/proto_value_conversions_unittest.cc",
     "syncable/directory_backing_store_unittest.cc",
     "syncable/directory_unittest.cc",