Update //third_party/protobuf to version 3.

third_party/protobuf/ruby/src/main/sentinel.proto

Index: third_party/protobuf/ruby/src/main/sentinel.proto
diff --git a/third_party/protobuf/ruby/src/main/sentinel.proto b/third_party/protobuf/ruby/src/main/sentinel.proto
new file mode 100644
index 0000000000000000000000000000000000000000..89a1ae1910d62c438bc3574fd66af763f107fc71
--- /dev/null
+++ b/third_party/protobuf/ruby/src/main/sentinel.proto
@@ -0,0 +1,15 @@
+syntax = "proto3";
+package com.google.protobuf.jruby;
+option optimize_for = CODE_SIZE;
+
+message Sentinel {
+  optional int32 default_int32 = 1;
+  optional int64 default_int64 = 2;
+  optional uint32 default_unit32 = 3;
+  optional uint64 default_uint64 = 4;
+  optional string default_string = 5;
+  optional bool default_bool = 6;
+  optional float default_float = 7;
+  optional double default_double = 8;
+  optional bytes default_bytes = 9;
+}