third_party/protobuf: Update to HEAD (83d681ee2c)

third_party/protobuf/csharp/src/Google.Protobuf/Reflection/PartialClasses.cs

 #region Copyright notice and license
 // Protocol Buffers - Google's data interchange format
 // Copyright 2008 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 39 matching lines @@
         // We can't tell the difference between "explicitly set to false" and "not set"
         // in proto3, but we need to tell the difference for FieldDescriptor.IsPacked.
         // This won't work if we ever need to support proto2, but at that point we'll be
         // able to remove this hack and use field presence instead. 
         partial void OnConstruction()
         {
             Packed = true;
         }
     }
 }