Update //third_party/protobuf to version 3.

third_party/protobuf/csharp/src/Google.Protobuf/Reflection/SingleFieldAccessor.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
+// 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
+
+using System;
+using System.Reflection;
+using Google.Protobuf.Compatibility;
+
+namespace Google.Protobuf.Reflection
+{
+    /// <summary>
+    /// Accessor for single fields.
+    /// </summary>
+    internal sealed class SingleFieldAccessor : FieldAccessorBase
+    {
+        // All the work here is actually done in the constructor - it creates the appropriate delegates.
+        // There are various cases to consider, based on the property type (message, string/bytes, or "genuine" primitive)
+        // and proto2 vs proto3 for non-message types, as proto3 doesn't support "full" presence detection or default
+        // values.
+
+        private readonly Action<IMessage, object> setValueDelegate;
+        private readonly Action<IMessage> clearDelegate;
+
+        internal SingleFieldAccessor(PropertyInfo property, FieldDescriptor descriptor) : base(property, descriptor)
+        {
+            if (!property.CanWrite)
+            {
+                throw new ArgumentException("Not all required properties/methods available");
+            }
+            setValueDelegate = ReflectionUtil.CreateActionIMessageObject(property.GetSetMethod());
+
+            var clrType = property.PropertyType;
+            
+            // TODO: Validate that this is a reasonable single field? (Should be a value type, a message type, or string/ByteString.)
+            object defaultValue =
+                descriptor.FieldType == FieldType.Message ? null
+                : clrType == typeof(string) ? ""
+                : clrType == typeof(ByteString) ? ByteString.Empty
+                : Activator.CreateInstance(clrType);
+            clearDelegate = message => SetValue(message, defaultValue);
+        }
+
+        public override void Clear(IMessage message)
+        {
+            clearDelegate(message);
+        }
+
+        public override void SetValue(IMessage message, object value)
+        {
+            setValueDelegate(message, value);
+        }
+    }
+}