third_party/protobuf: Update to HEAD (83d681ee2c)

third_party/protobuf/benchmarks/benchmark_messages_proto2.proto

Index: third_party/protobuf/benchmarks/benchmark_messages_proto2.proto
diff --git a/third_party/protobuf/benchmarks/benchmark_messages_proto2.proto b/third_party/protobuf/benchmarks/benchmark_messages_proto2.proto
index 01f67a1af3f2e5ae39288823c4a353454b1470cd..590855033cc08a88180637f20139780f83967b91 100644
--- a/third_party/protobuf/benchmarks/benchmark_messages_proto2.proto
+++ b/third_party/protobuf/benchmarks/benchmark_messages_proto2.proto
@@ -8,6 +8,8 @@ option java_package = "com.google.protobuf.benchmarks";
 // This is the default, but we specify it here explicitly.
 option optimize_for = SPEED;
 
+option cc_enable_arenas = true;
+
 message GoogleMessage1 {
   required string field1 = 1;
   optional string field9 = 9;