third_party/protobuf: Update to HEAD (83d681ee2c)

third_party/protobuf/csharp/src/Google.Protobuf/CodedOutputStream.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 741 matching lines @@
                 else
                 {
                     throw new InvalidOperationException(
                         "SpaceLeft can only be called on CodedOutputStreams that are " +
                         "writing to a flat array.");
                 }
             }
         }
     }
 }