Update //third_party/protobuf to version 3.

third_party/protobuf/python/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 the Python Protocol Buffers runtime library.
 
 Normally, this directory comes as part of the protobuf package, available
 from:
 
-  http://code.google.com/p/protobuf
+  https://developers.google.com/protocol-buffers/
 
 The complete package includes the C++ source code, which includes the
 Protocol Compiler (protoc).  If you downloaded this package from PyPI
 or some other Python-specific source, you may have received only the
 Python part of the code.  In this case, you will need to obtain the
 Protocol Compiler from some other source before you can use this
 package.
 
 Development Warning
 ===================
 
 The Python implementation of Protocol Buffers is not as mature as the C++
 and Java implementations.  It may be more buggy, and it is known to be
 pretty slow at this time.  If you would like to help fix these issues,
 join the Protocol Buffers discussion list and let us know!
 
 Installation
 ============
 
-1) Make sure you have Python 2.4 or newer.  If in doubt, run:
+1) Make sure you have Python 2.6 or newer.  If in doubt, run:
 
      $ python -V
 
 2) If you do not have setuptools installed, note that it will be
    downloaded and installed automatically as soon as you run setup.py.
    If you would rather install it manually, you may do so by following
    the instructions on this page:
 
-     http://peak.telecommunity.com/DevCenter/EasyInstall#installation-instructions
+     https://packaging.python.org/en/latest/installing.html#setup-for-installing-packages
 
 3) Build the C++ code, or install a binary distribution of protoc.  If
    you install a binary distribution, make sure that it is the same
    version as this package.  If in doubt, run:
 
      $ protoc --version
 
-4) Run the tests:
+4) Build and run the tests:
 
+     $ python setup.py build
      $ python setup.py test
 
+     To build, test, and use the C++ implementation, you must first compile
+     libprotobuf.so:
+
+     $ (cd .. && make)
+
+     On OS X:
+
+      If you are running a homebrew-provided python, you must make sure another
+      version of protobuf is not already installed, as homebrew's python will
+      search /usr/local/lib for libprotobuf.so before it searches ../src/.libs
+      You can either unlink homebrew's protobuf or install the libprotobuf you
+      built earlier:
+
+      $ brew unlink protobuf
+      or
+      $ (cd .. && make install)
+
+     On other *nix:
+
+      You must make libprotobuf.so dynamically available. You can either
+      install libprotobuf you built earlier, or set LD_LIBRARY_PATH:
+
+      $ export LD_LIBRARY_PATH=../src/.libs
+      or
+      $ (cd .. && make install)
+
+     To build the C++ implementation run:
+     $ python setup.py build --cpp_implementation
+
+     Then run the tests like so:
+     $ python setup.py test --cpp_implementation
+
    If some tests fail, this library may not work correctly on your
    system.  Continue at your own risk.
 
    Please note that there is a known problem with some versions of
    Python on Cygwin which causes the tests to fail after printing the
    error:  "sem_init: Resource temporarily unavailable".  This appears
    to be a bug either in Cygwin or in Python:
      http://www.cygwin.com/ml/cygwin/2005-07/msg01378.html
    We do not know if or when it might me fixed.  We also do not know
    how likely it is that this bug will affect users in practice.
 
 5) Install:
 
-     $ python setup.py install
+    $ python setup.py install
+
+  or:
+
+    $ (cd .. && make install)
+    $ python setup.py install --cpp_implementation
 
    This step may require superuser privileges.
-   NOTE: To use C++ implementation, you need to install C++ protobuf runtime
-   library of the same version and export the environment variable before this
-   step. See the "C++ Implementation" section below for more details.
+   NOTE: To use C++ implementation, you need to export an environment
+   variable before running your program.  See the "C++ Implementation"
+   section below for more details.
 
 Usage
 =====
 
 The complete documentation for Protocol Buffers is available via the
 web at:
 
-  http://code.google.com/apis/protocolbuffers/
+  https://developers.google.com/protocol-buffers/
 
 C++ Implementation
 ==================
 
-WARNING: This is EXPERIMENTAL and only available for CPython platforms.
-
 The C++ implementation for Python messages is built as a Python extension to
 improve the overall protobuf Python performance.
 
 To use the C++ implementation, you need to:
 1) Install the C++ protobuf runtime library, please see instructions in the
    parent directory.
 2) Export an environment variable:
 
-  $ export PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=cpp
+    $ export PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=cpp
 
-You need to export this variable before running setup.py script to build and
-install the extension.  You must also set the variable at runtime, otherwise
+You must set this variable at runtime, before running your program, otherwise
 the pure-Python implementation will be used. In a future release, we will
 change the default so that C++ implementation is used whenever it is available.
-It is strongly recommended to run `python setup.py test` after setting the
-variable to "cpp", so the tests will be against C++ implemented Python
-messages.
-