Update //third_party/protobuf to version 3.

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

+#region Copyright notice and license
 // Protocol Buffers - Google's data interchange format
-// Copyright 2008 Google Inc.  All rights reserved.
-// http://code.google.com/p/protobuf/
+// 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
 // notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 // copyright notice, this list of conditions and the following disclaimer
 // in the documentation and/or other materials provided with the
 // distribution.
 //     * Neither the name of Google Inc. nor the names of its
 // contributors may be used to endorse or promote products derived from
 // this software without specific prior written permission.
 //
 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#endregion
 
-// Author: jasonh@google.com (Jason Hsueh)
-//
-// This file is used to test a corner case in the CPP implementation where the
-// generated C++ type is available for the extendee, but the extension is
-// defined in a file whose C++ type is not in the binary.
+using System;
+using System.Collections;
+using System.Reflection;
 
+namespace Google.Protobuf.Reflection
+{
+    /// <summary>
+    /// Accessor for map fields.
+    /// </summary>
+    internal sealed class MapFieldAccessor : FieldAccessorBase
+    {
+        internal MapFieldAccessor(PropertyInfo property, FieldDescriptor descriptor) : base(property, descriptor)
+        {
+        }
 
-import "google/protobuf/internal/more_extensions.proto";
+        public override void Clear(IMessage message)
+        {
+            IDictionary list = (IDictionary) GetValue(message);
+            list.Clear();
+        }
 
-package google.protobuf.internal;
-
-message DynamicMessageType {
-  optional int32 a = 1;
+        public override void SetValue(IMessage message, object value)
+        {
+            throw new InvalidOperationException("SetValue is not implemented for map fields");
+        }
+    }
 }
-
-extend ExtendedMessage {
-  optional int32 dynamic_int32_extension = 100;
-  optional DynamicMessageType dynamic_message_extension = 101;
-}