Update //third_party/protobuf to version 3.

third_party/protobuf/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) [![Build status](https://ci.appveyor.com/api/projects/status/73ctee6ua4w2ruin?svg=true)](https://ci.appveyor.com/project/protobuf/protobuf)
+
 Copyright 2008 Google Inc.
-http://code.google.com/apis/protocolbuffers/
+
+https://developers.google.com/protocol-buffers/
 
 C++ Installation - Unix
-=======================
+-----------------------
+
+If you get the source from github, you need to generate the configure script
+first:
+
+    $ ./autogen.sh
+
+This will download gmock source (which is used for C++ Protocol Buffer
+unit-tests) to the current directory and run automake, autoconf, etc.
+to generate the configure script and various template makefiles.
+
+You can skip this step if you are using a release package (which already
+contains gmock and the configure script).
 
 To build and install the C++ Protocol Buffer runtime and the Protocol
 Buffer compiler (protoc) execute the following:
 
-  $ ./configure
-  $ make
-  $ make check
-  $ make install
+    $ ./configure
+    $ make
+    $ make check
+    $ make install
 
 If "make check" fails, you can still install, but it is likely that
 some features of this library will not work correctly on your system.
 Proceed at your own risk.
 
 "make install" may require superuser privileges.
 
 For advanced usage information on configure and make, see INSTALL.txt.
 
-** Hint on install location **
+**Hint on install location**
 
   By default, the package will be installed to /usr/local.  However,
   on many platforms, /usr/local/lib is not part of LD_LIBRARY_PATH.
   You can add it, but it may be easier to just install to /usr
   instead.  To do this, invoke configure as follows:
 
     ./configure --prefix=/usr
 
   If you already built the package with a different prefix, make sure
   to run "make clean" before building again.
 
-** Compiling dependent packages **
+**Compiling dependent packages**
 
   To compile a package that uses Protocol Buffers, you need to pass
   various flags to your compiler and linker.  As of version 2.2.0,
   Protocol Buffers integrates with pkg-config to manage this.  If you
   have pkg-config installed, then you can invoke it to get a list of
   flags like so:
 
     pkg-config --cflags protobuf         # print compiler flags
     pkg-config --libs protobuf           # print linker flags
     pkg-config --cflags --libs protobuf  # print both
@@ skipping 17 matching lines @@
   Buffers, you should probably use the PKG_CHECK_MODULES macro in your
   configure script like:
 
     PKG_CHECK_MODULES([protobuf], [protobuf])
 
   See the pkg-config man page for more info.
 
   If you only want protobuf-lite, substitute "protobuf-lite" in place
   of "protobuf" in these examples.
 
-** Note for cross-compiling **
+**Note for Mac users**
+
+  For a Mac system, Unix tools are not available by default. You will first need
+  to install Xcode from the Mac AppStore and then run the following command from
+  a terminal:
+
+    $ sudo xcode-select --install
+
+  To install Unix tools, you can install "port" following the instructions at
+  https://www.macports.org . This will reside in /opt/local/bin/port for most
+  Mac installations.
+
+    $ sudo /opt/local/bin/port install autoconf automake libtool
+
+  Then follow the Unix instructions above.
+
+**Note for cross-compiling**
 
   The makefiles normally invoke the protoc executable that they just
   built in order to build tests.  When cross-compiling, the protoc
   executable may not be executable on the host machine.  In this case,
   you must build a copy of protoc for the host machine first, then use
   the --with-protoc option to tell configure to use it instead.  For
   example:
 
     ./configure --with-protoc=protoc
 
   This will use the installed protoc (found in your $PATH) instead of
   trying to execute the one built during the build process.  You can
   also use an executable that hasn't been installed.  For example, if
   you built the protobuf package for your host machine in ../host,
   you might do:
 
     ./configure --with-protoc=../host/src/protoc
 
   Either way, you must make sure that the protoc executable you use
   has the same version as the protobuf source code you are trying to
   use it with.
 
-** Note for Solaris users **
+**Note for Solaris users**
 
   Solaris 10 x86 has a bug that will make linking fail, complaining
   about libstdc++.la being invalid.  We have included a work-around
   in this package.  To use the work-around, run configure as follows:
 
     ./configure LDFLAGS=-L$PWD/src/solaris
 
   See src/solaris/libstdc++.la for more info on this bug.
 
-** Note for HP C++ Tru64 users **
+**Note for HP C++ Tru64 users**
 
   To compile invoke configure as follows:
 
     ./configure CXXFLAGS="-O -std ansi -ieee -D__USE_STD_IOSTREAM"
 
   Also, you will need to use gmake instead of make.
 
+**Note for AIX users**
+
+  Compile using the IBM xlC C++ compiler as follows:
+
+    ./configure CXX=xlC
+
+  Also, you will need to use GNU `make` (`gmake`) instead of AIX `make`.
+
 C++ Installation - Windows
-==========================
+--------------------------
 
-If you are using Microsoft Visual C++, see vsprojects/readme.txt.
+If you are using Microsoft Visual C++, see cmake/README.md.
 
 If you are using Cygwin or MinGW, follow the Unix installation
 instructions, above.
 
 Binary Compatibility Warning
-============================
+----------------------------
 
 Due to the nature of C++, it is unlikely that any two versions of the
 Protocol Buffers C++ runtime libraries will have compatible ABIs.
 That is, if you linked an executable against an older version of
 libprotobuf, it is unlikely to work with a newer version without
 re-compiling.  This problem, when it occurs, will normally be detected
 immediately on startup of your app.  Still, you may want to consider
 using static linkage.  You can configure this package to install
 static libraries only using:
 
-  ./configure --disable-shared
+    ./configure --disable-shared
 
 Java and Python Installation
-============================
+----------------------------
 
 The Java and Python runtime libraries for Protocol Buffers are located
 in the java and python directories.  See the README file in each
 directory for more information on how to compile and install them.
 Note that both of them require you to first install the Protocol
 Buffer compiler (protoc), which is part of the C++ package.
 
 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/