Update //third_party/protobuf to version 3.

third_party/protobuf/ruby/tests/generated_code.proto

+syntax = "proto3";
+
+package A.B.C;
+
+message TestMessage {
+  int32 optional_int32 = 1;
+  int64 optional_int64 = 2;
+  uint32 optional_uint32 = 3;
+  uint64 optional_uint64 = 4;
+  bool optional_bool = 5;
+  double optional_double = 6;
+  float optional_float = 7;
+  string optional_string = 8;
+  bytes optional_bytes = 9;
+  TestEnum optional_enum = 10;
+  TestMessage optional_msg = 11;
+
+  repeated int32 repeated_int32 = 21;
+  repeated int64 repeated_int64 = 22;
+  repeated uint32 repeated_uint32 = 23;
+  repeated uint64 repeated_uint64 = 24;
+  repeated bool repeated_bool = 25;
+  repeated double repeated_double = 26;
+  repeated float repeated_float = 27;
+  repeated string repeated_string = 28;
+  repeated bytes repeated_bytes = 29;
+  repeated TestEnum repeated_enum = 30;
+  repeated TestMessage repeated_msg = 31;
+
+  oneof my_oneof {
+    int32 oneof_int32 = 41;
+    int64 oneof_int64 = 42;
+    uint32 oneof_uint32 = 43;
+    uint64 oneof_uint64 = 44;
+    bool oneof_bool = 45;
+    double oneof_double = 46;
+    float oneof_float = 47;
+    string oneof_string = 48;
+    bytes oneof_bytes = 49;
+    TestEnum oneof_enum = 50;
+    TestMessage oneof_msg = 51;
+  }
+
+  map<int32, string> map_int32_string = 61;
+  map<int64, string> map_int64_string = 62;
+  map<uint32, string> map_uint32_string = 63;
+  map<uint64, string> map_uint64_string = 64;
+  map<bool, string> map_bool_string = 65;
+  map<string, string> map_string_string = 66;
+  map<string, TestMessage> map_string_msg = 67;
+  map<string, TestEnum> map_string_enum = 68;
+  map<string, int32> map_string_int32 = 69;
+  map<string, bool> map_string_bool = 70;
+
+  message NestedMessage {
+    int32 foo = 1;
+  }
+
+  NestedMessage nested_message = 80;
+}
+
+enum TestEnum {
+  Default = 0;
+  A = 1;
+  B = 2;
+  C = 3;
+}