Replace kEmptyString with OnceInit initialized string*

third_party/protobuf/src/google/protobuf/repeated_field.h

Index: third_party/protobuf/src/google/protobuf/repeated_field.h
diff --git a/third_party/protobuf/src/google/protobuf/repeated_field.h b/third_party/protobuf/src/google/protobuf/repeated_field.h
index 570d4b751297bbff7c9aeff4bcbc08dde71034e0..ff5d1f4b5f40d65ab75d0107f7ff72c99744571c 100644
--- a/third_party/protobuf/src/google/protobuf/repeated_field.h
+++ b/third_party/protobuf/src/google/protobuf/repeated_field.h
@@ -383,7 +383,7 @@ class LIBPROTOBUF_EXPORT StringTypeHandlerBase {
   static void Clear(string* value) { value->clear(); }
   static void Merge(const string& from, string* to) { *to = from; }
   static const Type& default_instance() {
-    return ::google::protobuf::internal::kEmptyString;
+    return ::google::protobuf::internal::GetEmptyString();
   }
 };