third_party/protobuf: Update to HEAD (83d681ee2c)

third_party/protobuf/benchmarks/benchmark_messages_proto2.proto

 // Benchmark messages for proto2.
 
 syntax = "proto2";
 
 package benchmarks.proto2;
 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;
   optional string field18 = 18;
   optional bool field80 = 80 [default=false];
   optional bool field81 = 81 [default=true];
   required int32 field2 = 2;
   required int32 field3 = 3;
   optional int32 field280 = 280;
   optional int32 field6 = 6 [default=0];
@@ skipping 111 matching lines @@
   optional float field3 = 3 [default=0.0];
   optional bool field4 = 4;
   optional bool field5 = 5;
   optional bool field6 = 6 [default=true];
   optional bool field7 = 7 [default=false];
   optional float field8 = 8;
   optional bool field9 = 9;
   optional float field10 = 10;
   optional int64 field11 = 11;
 }