sync: remove use of protobuf extensions in protocol to reduce static init overhead.

chrome/browser/sync/protocol/proto_value_conversions_unittest.cc

Index: chrome/browser/sync/protocol/proto_value_conversions_unittest.cc
diff --git a/chrome/browser/sync/protocol/proto_value_conversions_unittest.cc b/chrome/browser/sync/protocol/proto_value_conversions_unittest.cc
index de61897de7eff732a895d40cc7df2a5acc7bf5ed..31f1608bf817d5f906876f5141761bf6be8f923e 100644
--- a/chrome/browser/sync/protocol/proto_value_conversions_unittest.cc
+++ b/chrome/browser/sync/protocol/proto_value_conversions_unittest.cc
@@ -162,7 +162,7 @@ TEST_F(ProtoValueConversionsTest, EntitySpecificsToValue) {
   sync_pb::EntitySpecifics specifics;
   // Touch the extensions to make sure it shows up in the generated
   // value.
-#define SET_EXTENSION(key) (void)specifics.MutableExtension(sync_pb::key)
+#define SET_EXTENSION(key) (void)specifics.mutable_##key()
 
   SET_EXTENSION(app);
   SET_EXTENSION(app_notification);