Reverts third_party/protobuf: Update to HEAD (f52e188fe4)

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

 #region Copyright notice and license
 // Protocol Buffers - Google's data interchange format
 // Copyright 2015 Google Inc.  All rights reserved.
 // https://developers.google.com/protocol-buffers/
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
@@ skipping 329 matching lines @@
     /// API is very limited - just what the generated code needs to call directly.
     /// </para>
     /// </summary>
     /// <remarks>
     /// This never writes default values to the stream, and does not address "packedness"
     /// in repeated fields itself, other than to know whether or not the field *should* be packed.
     /// </remarks>
     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()
         {
             if (typeof(T) == typeof(string))
             {
                 DefaultDefault = (T)(object)"";
             }
             else if (typeof(T) == typeof(ByteString))
             {
                 DefaultDefault = (T)(object)ByteString.Empty;
@@ skipping 103 matching lines @@
 
         /// <summary>
         /// Calculates the size required to write the given value, with a tag,
         /// if the value is not the default.
         /// </summary>
         public int CalculateSizeWithTag(T value) => IsDefault(value) ? 0 : ValueSizeCalculator(value) + tagSize;
 
         private bool IsDefault(T value) => EqualityComparer<T>.Default.Equals(value, DefaultValue);
     }
 }