Update //third_party/protobuf to version 3.

third_party/protobuf/README.txt

-Protocol Buffers - Google's data interchange format
-Copyright 2008 Google Inc.
-http://code.google.com/apis/protocolbuffers/
-
-C++ Installation - Unix
-=======================
-
-To build and install the C++ Protocol Buffer runtime and the Protocol
-Buffer compiler (protoc) execute the following:
-
-  $ ./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 **
-
-  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 **
-
-  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
-
-  For example:
-
-    c++ my_program.cc my_proto.pb.cc `pkg-config --cflags --libs protobuf`
-
-  Note that packages written prior to the 2.2.0 release of Protocol
-  Buffers may not yet integrate with pkg-config to get flags, and may
-  not pass the correct set of flags to correctly link against
-  libprotobuf.  If the package in question uses autoconf, you can
-  often fix the problem by invoking its configure script like:
-
-    configure CXXFLAGS="$(pkg-config --cflags protobuf)" \
-              LIBS="$(pkg-config --libs protobuf)"
-
-  This will force it to use the correct flags.
-
-  If you are writing an autoconf-based package that uses Protocol
-  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 **
-
-  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 $PATH) instead of
-  trying to execute the one built during the build process.  You can
-  also use an executable that hasn't been installed.  For example, if
-  you built the protobuf package for your host machine in ../host,
-  you might do:
-
-    ./configure --with-protoc=../host/src/protoc
-
-  Either way, you must make sure that the protoc executable you use
-  has the same version as the protobuf source code you are trying to
-  use it with.
-
-** Note for Solaris users **
-
-  Solaris 10 x86 has a bug that will make linking fail, complaining
-  about libstdc++.la being invalid.  We have included a work-around
-  in this package.  To use the work-around, run configure as follows:
-
-    ./configure LDFLAGS=-L$PWD/src/solaris
-
-  See src/solaris/libstdc++.la for more info on this bug.
-
-** Note for HP C++ Tru64 users **
-
-  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.
-
-C++ Installation - Windows
-==========================
-
-If you are using Microsoft Visual C++, see vsprojects/readme.txt.
-
-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
-
-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/