Update //third_party/protobuf to version 3.

third_party/protobuf/examples/addressbook.proto

+// See README.txt for information and build instructions.
+//
+// Note: START and END tags are used in comments to define sections used in
+// tutorials.  They are not part of the syntax for Protocol Buffers.
+//
+// To get an in-depth walkthrough of this file and the related examples, see:
+// https://developers.google.com/protocol-buffers/docs/tutorials
+
+// [START declaration]
+syntax = "proto3";
+package tutorial;
+// [END declaration]
+
+// [START java_declaration]
+option java_package = "com.example.tutorial";
+option java_outer_classname = "AddressBookProtos";
+// [END java_declaration]
+
+// [START csharp_declaration]
+option csharp_namespace = "Google.Protobuf.Examples.AddressBook";
+// [END csharp_declaration]
+
+// [START messages]
+message Person {
+  string name = 1;
+  int32 id = 2;  // Unique ID number for this person.
+  string email = 3;
+
+  enum PhoneType {
+    MOBILE = 0;
+    HOME = 1;
+    WORK = 2;
+  }
+
+  message PhoneNumber {
+    string number = 1;
+    PhoneType type = 2;
+  }
+
+  repeated PhoneNumber phones = 4;
+}
+
+// Our address book file is just one of these.
+message AddressBook {
+  repeated Person people = 1;
+}
+// [END messages]