Update //third_party/protobuf to version 3.

third_party/protobuf/python/google/protobuf/internal/more_extensions_dynamic.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
@@ skipping 13 matching lines @@
 // 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: 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.
 
+syntax = "proto2";
 
 import "google/protobuf/internal/more_extensions.proto";
 
 package google.protobuf.internal;
 
 message DynamicMessageType {
   optional int32 a = 1;
 }
 
 extend ExtendedMessage {
   optional int32 dynamic_int32_extension = 100;
   optional DynamicMessageType dynamic_message_extension = 101;
 }