Update //third_party/protobuf to version 3.

third_party/protobuf/src/google/protobuf/empty.proto

 // Protocol Buffers - Google's data interchange format
 // Copyright 2008 Google Inc.  All rights reserved.
-// http://code.google.com/p/protobuf/
+// 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.
 
-// Author: kenton@google.com (Kenton Varda)
+syntax = "proto3";
+
+package google.protobuf;
+
+option csharp_namespace = "Google.Protobuf.WellKnownTypes";
+option java_package = "com.google.protobuf";
+option java_outer_classname = "EmptyProto";
+option java_multiple_files = true;
+option java_generate_equals_and_hash = true;
+option objc_class_prefix = "GPB";
+option cc_enable_arenas = true;
+
+// A generic empty message that you can re-use to avoid defining duplicated
+// empty messages in your APIs. A typical example is to use it as the request
+// or the response type of an API method. For instance:
 //
-// This is like unittest_import.proto but with optimize_for = LITE_RUNTIME.
-
-package protobuf_unittest_import;
-
-option optimize_for = LITE_RUNTIME;
-
-option java_package = "com.google.protobuf";
-
-import public "google/protobuf/unittest_import_public_lite.proto";
-
-message ImportMessageLite {
-  optional int32 d = 1;
-}
-
-enum ImportEnumLite {
-  IMPORT_LITE_FOO = 7;
-  IMPORT_LITE_BAR = 8;
-  IMPORT_LITE_BAZ = 9;
-}
+//     service Foo {
+//       rpc Bar(google.protobuf.Empty) returns (google.protobuf.Empty);
+//     }
+//
+// The JSON representation for `Empty` is empty JSON object `{}`.
+message Empty {}