Update //third_party/protobuf to version 3.

third_party/protobuf/vsprojects/readme.txt

-This directory contains project files for compiling Protocol Buffers using
-MSVC.  This is not the recommended way to do Protocol Buffer development --
-we prefer to develop under a Unix-like environment -- but it may be more
-accessible to those who primarily work with MSVC.
-
-Compiling and Installing
-========================
-
-1) Open protobuf.sln in Microsoft Visual Studio.
-2) Choose "Debug" or "Release" configuration as desired.*
-3) From the Build menu, choose "Build Solution".  Wait for compiling to finish.
-4) From a command shell, run tests.exe and lite-test.exe and check that all
-   tests pass.
-5) Run extract_includes.bat to copy all the public headers into a separate
-   "include" directory (under the top-level package directory).
-6) Copy the contents of the include directory to wherever you want to put
-   headers.
-7) Copy protoc.exe wherever you put build tools (probably somewhere in your
-   PATH).
-8) Copy libprotobuf.lib, libprotobuf-lite.lib, and libprotoc.lib wherever you
-   put libraries.
-
-* To avoid conflicts between the MSVC debug and release runtime libraries, when
-  compiling a debug build of your application, you may need to link against a
-  debug build of libprotobuf.lib.  Similarly, release builds should link against
-  release libs.
-
-DLLs vs. static linking
-=======================
-
-Static linking is now the default for the Protocol Buffer libraries.  Due to
-issues with Win32's use of a separate heap for each DLL, as well as binary
-compatibility issues between different versions of MSVC's STL library, it is
-recommended that you use static linkage only.  However, it is possible to
-build libprotobuf and libprotoc as DLLs if you really want.  To do this,
-do the following:
-
-  1) Open protobuf.sln in MSVC.
-  2) For each of the projects libprotobuf, libprotobuf-lite, and libprotoc, do
-     the following:
-    2a) Right-click the project and choose "properties".
-    2b) From the side bar, choose "General", under "Configuration Properties".
-    2c) Change the "Configuration Type" to "Dynamic Library (.dll)".
-    2d) From the side bar, choose "Preprocessor", under "C/C++".
-    2e) Add PROTOBUF_USE_DLLS to the list of preprocessor defines.
-  3) When compiling your project, make sure to #define PROTOBUF_USE_DLLS.
-
-When distributing your software to end users, we strongly recommend that you
-do NOT install libprotobuf.dll or libprotoc.dll to any shared location.
-Instead, keep these libraries next to your binaries, in your application's
-own install directory.  C++ makes it very difficult to maintain binary
-compatibility between releases, so it is likely that future versions of these
-libraries will *not* be usable as drop-in replacements.
-
-If your project is itself a DLL intended for use by third-party software, we
-recommend that you do NOT expose protocol buffer objects in your library's
-public interface, and that you statically link protocol buffers into your
-library.
-
-ZLib support
-============
-
-If you want to include GzipInputStream and GzipOutputStream
-(google/protobuf/io/gzip_stream.h) in libprotoc, you will need to do a few
-additional steps:
-
-1) Obtain a copy of the zlib library.  The pre-compiled DLL at zlib.net works.
-2) Make sure zlib's two headers are in your include path and that the .lib file
-   is in your library path.  You could place all three files directly into the
-   vsproject directory to compile libprotobuf, but they need to be visible to
-   your own project as well, so you should probably just put them into the
-   VC shared icnlude and library directories.
-3) Right-click on the "tests" project and choose "properties".  Navigate the
-   sidebar to "Configuration Properties" -> "Linker" -> "Input".
-4) Under "Additional Dependencies", add the name of the zlib .lib file (e.g.
-   zdll.lib).  Make sure to update both the Debug and Release configurations.
-5) If you are compiling libprotobuf and libprotoc as DLLs (see previous
-   section), repeat steps 2 and 3 for the libprotobuf and libprotoc projects.
-   If you are compiling them as static libraries, then you will need to link
-   against the zlib library directly from your own app.
-6) Edit config.h (in the vsprojects directory) and un-comment the line that
-   #defines HAVE_ZLIB.  (Or, alternatively, define this macro via the project
-   settings.)
-
-Notes on Compiler Warnings
-==========================
-
-The following warnings have been disabled while building the protobuf libraries
-and compiler.  You may have to disable some of them in your own project as
-well, or live with them.
-
-C4018 - 'expression' : signed/unsigned mismatch
-C4146 - unary minus operator applied to unsigned type, result still unsigned
-C4244 - Conversion from 'type1' to 'type2', possible loss of data.
-C4251 - 'identifier' : class 'type' needs to have dll-interface to be used by
-        clients of class 'type2'
-C4267 - Conversion from 'size_t' to 'type', possible loss of data.
-C4305 - 'identifier' : truncation from 'type1' to 'type2'
-C4355 - 'this' : used in base member initializer list
-C4800 - 'type' : forcing value to bool 'true' or 'false' (performance warning)
-C4996 - 'function': was declared deprecated
-
-C4251 is of particular note, if you are compiling the Protocol Buffer library
-as a DLL (see previous section).  The protocol buffer library uses templates in
-its public interfaces.  MSVC does not provide any reasonable way to export
-template classes from a DLL.  However, in practice, it appears that exporting
-templates is not necessary anyway.  Since the complete definition of any
-template is available in the header files, anyone importing the DLL will just
-end up compiling instances of the templates into their own binary.  The
-Protocol Buffer implementation does not rely on static template members being
-unique, so there should be no problem with this, but MSVC prints warning
-nevertheless.  So, we disable it.  Unfortunately, this warning will also be
-produced when compiling code which merely uses protocol buffers, meaning you
-may have to disable it in your code too.