Update //third_party/protobuf to version 3.

third_party/protobuf/conformance/README.md

+Protocol Buffers - Google's data interchange format
+===================================================
+
+[![Build Status](https://travis-ci.org/google/protobuf.svg?branch=master)](https://travis-ci.org/google/protobuf)
+
+Copyright 2008 Google Inc.
+
+This directory contains conformance tests for testing completeness and
+correctness of Protocol Buffers implementations.  These tests are designed
+to be easy to run against any Protocol Buffers implementation.
+
+This directory contains the tester process `conformance-test`, which
+contains all of the tests themselves.  Then separate programs written
+in whatever language you want to test communicate with the tester
+program over a pipe.
+
+Before running any of these tests, make sure you run `make` in the base
+directory to build `protoc`, since all the tests depend on it.
+
+    $ make
+
+Then to run the tests against the C++ implementation, run:
+
+    $ cd conformance && make test_cpp
+
+More tests and languages will be added soon!
+
+Testing other Protocol Buffer implementations
+---------------------------------------------
+
+To run these tests against a new Protocol Buffers implementation, write a
+program in your language that uses the protobuf implementation you want
+to test.  This program should implement the testing protocol defined in
+[conformance.proto](https://github.com/google/protobuf/blob/master/conformance/conformance.proto).
+This is designed to be as easy as possible: the C++ version is only
+150 lines and is a good example for what this program should look like
+(see [conformance_cpp.cc](https://github.com/google/protobuf/blob/master/conformance/conformance_cpp.cc)).
+The program only needs to be able to read from stdin and write to stdout.
+
+Portability
+-----------
+
+Note that the test runner currently does not work on Windows.  Patches
+to fix this are welcome!  (But please get in touch first to settle on
+a general implementation strategy).