[Sync] Reimplement proto value conversions on top of field visitors.

components/sync/BUILD.gn

Index: components/sync/BUILD.gn
diff --git a/components/sync/BUILD.gn b/components/sync/BUILD.gn
index 0535e22b5011f6897ccf82f53602f3d714a63696..49ced459b2d459d0082122d44167bdedec424c4e 100644
--- a/components/sync/BUILD.gn
+++ b/components/sync/BUILD.gn
@@ -451,6 +451,8 @@ static_library("sync") {
     "model_impl/shared_model_type_processor.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") {
     "model_impl/model_type_store_impl_unittest.cc",
     "model_impl/processor_entity_tracker_unittest.cc",
     "model_impl/shared_model_type_processor_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/change_record_unittest.cc",
     "syncable/directory_backing_store_unittest.cc",