Revert "third_party/protobuf: Update to HEAD (83d681ee2c)"

third_party/protobuf/csharp/src/Google.Protobuf/FieldCodec.cs

Index: third_party/protobuf/csharp/src/Google.Protobuf/FieldCodec.cs
diff --git a/third_party/protobuf/csharp/src/Google.Protobuf/FieldCodec.cs b/third_party/protobuf/csharp/src/Google.Protobuf/FieldCodec.cs
index c28b47e1eb76f327d3461623db6f6290da850091..9831308882b08e9fdd7b67bdd38b3c48c06d3dc2 100644
--- a/third_party/protobuf/csharp/src/Google.Protobuf/FieldCodec.cs
+++ b/third_party/protobuf/csharp/src/Google.Protobuf/FieldCodec.cs
@@ -347,8 +347,7 @@ namespace Google.Protobuf
     public sealed class FieldCodec<T>
     {
         private static readonly T DefaultDefault;
-        // Only non-nullable value types support packing. This is the simplest way of detecting that.
-        private static readonly bool TypeSupportsPacking = default(T) != null;
+        private static readonly bool TypeSupportsPacking = typeof(T).IsValueType() && Nullable.GetUnderlyingType(typeof(T)) == null;
 
         static FieldCodec()
         {