Pull new version of protobuf sources.

third_party/protobuf/src/google/protobuf/unittest_import.proto

Index: third_party/protobuf/src/google/protobuf/unittest_import.proto
diff --git a/third_party/protobuf/src/google/protobuf/unittest_import.proto b/third_party/protobuf/src/google/protobuf/unittest_import.proto
index c115b11171afaa03d7ee6aeedc7506689cfd7038..7e165220334f556556c21b881df20918b6218d9a 100644
--- a/third_party/protobuf/src/google/protobuf/unittest_import.proto
+++ b/third_party/protobuf/src/google/protobuf/unittest_import.proto
@@ -1,6 +1,6 @@
 // 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
@@ -34,6 +34,7 @@
 //
 // A proto file which is imported by unittest.proto to test importing.
 
+syntax = "proto2";
 
 // We don't put this in a package within proto2 because we need to make sure
 // that the generated code doesn't depend on being in the proto2 namespace.
@@ -42,8 +43,9 @@
 package protobuf_unittest_import;
 
 option optimize_for = SPEED;
+option cc_enable_arenas = true;
 
-// Excercise the java_package option.
+// Exercise the java_package option.
 option java_package = "com.google.protobuf.test";
 
 // Do not set a java_outer_classname here to verify that Proto2 works without