Update //third_party/protobuf to version 3.

third_party/protobuf/src/google/protobuf/unittest_import_lite.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 11 matching lines @@
 // 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)
 //
 // This is like unittest_import.proto but with optimize_for = LITE_RUNTIME.
 
+syntax = "proto2";
 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;
 }