Update //third_party/protobuf to version 3.

third_party/protobuf/README.chromium

 Name: Protocol Buffers
 Short Name: protobuf
-URL: http://protobuf.googlecode.com/svn/trunk
+URL: https://github.com/google/protobuf
 License: BSD
-License File: COPYING.txt
-Version: unknown
-Revision: r476
+License File: LICENSE
+Version: 3.0.0-beta-2
+Revision: d5fb408ddc281ffcadeb08699e65bb694656d0bd
 Security Critical: yes
 
-Local files (not taken from upstream):
-README.chromium
-config.h
-descriptor2_pb.py
+Steps used to create the current version:
+1. Pull the release from https://github.com/google/protobuf/releases
+2. Use the cleanup_release.sh script to remove unnecessary files.
+3. Add build files (BUILD.gn, proto_library.gni, protobuf.gyp,
+   protobuf_lite.gypi, protobuf_nacl.gyp).
 
-A protobuf.gyp file has been added for building with Chromium.
+   Be sure to update the list of source files, as additional .cc files and
+   headers might have been added -- you need to find the transitive closure of
+   include files required by targets.
 
-This code has been patched to support unknown field retention in protobuf-lite.
-See r62331 for the patch.
+   Other things to care about are defines required by protobuf on various
+   platforms, warnings generated by compilers, and new dependencies introduced.
+4. Get open-source library six.py from https://pypi.python.org/pypi/six/ and add
+   it to protobuf/third_party/six/six.py.
+5. Apply patches in patches/ (see the description below):
 
-This code has been patched to ensure that files in the target protobuf_lite
-do not include headers from protobuf_full. See r173228 for the patch.
+   $ for patch in patches/*; do patch -s -p1 < $patch; done
 
-This code has been patched to make the target protobuf_lite a component so that
-targets that depend on it can be componentized. See http://crbug.com/172800 for
-details, and r179806 for the patch.
+   For future releases, it will be worth looking into which patches still need
+   to be applied.
+5. Generate descriptor_pb2.py using something like the following steps. Make
+   sure you've regenerated your buildfiles and will build protoc from the
+   newly-modified sources above.
 
-Revision 504 was cherry-picked from upstream.
-Revision 512 was cherry-picked from upstream.
-Revision 516 was cherry-picked from upstream.
-Revision 517 was cherry-picked from upstream.
-Revision 522 was cherry-picked from upstream.
-Revision 523 was cherry-picked from upstream.
-Revision 524 was cherry-picked from upstream.
-Revision 573 was cherry-picked from upstream.
-The `&file->options() != NULL &&` was removed from descriptor.cc
-References to `vector` were changed to `std::vector` in extension_set.h
-and unknown_field_set.h
+   $ cd $SRC_DIR
+   $ ninja -C out/Debug protoc
+   $ cd third_party/protobuf/src
+   $ ../../../out/Debug/protoc --python_out=../python google/protobuf/descriptor.proto
 
-GetEmptyString() and GoogleOnceInit() have been uninlined, for a large savings
-in binary size.
+6. Add an __init__.py to protobuf/ that adds third_party/six/ to Python path.
+7. Update README.chromium.
 
-A BUILD.gn file has been added for building with GN.
+Description of the patches:
 
-Cherry-pick pherl changes to make protobuf build on VS2015.
+- 0001-ignore-option-retain-unknown-fields.patch
 
-Cherry-pick c3cb53b (fix "sometimes-uninitialized" warning).
+  Previous versions of protobuf in Chromium carried a local patch that retained
+  unknown fields in protobuf_lite mode. It was enabled by setting option
+  retain_unknown_fields = true in .proto file. Now that it is enabled by
+  default, this option is no longer recognized by protobuf, and so I had to
+  patch it so that I don't have to fix all .proto files in Chromium in a single
+  CL.
 
-Cherry-pick https://github.com/google/protobuf/commit/56a90a2081379a5
+  I plan to remove those occurences, and then this patch will no longer be
+  necessary.
 
-Cherry-pick https://github.com/google/protobuf/commit/69d660b39ceabea
+- 0003-remove-static-initializers.patch
 
-Cherry-pick https://github.com/google/protobuf/commit/a74e912a8be1274
+  This patch removes all static initializers from Chromium. The change in Status
+  class is not completely compatible with upstream, but it's compatible enough
+  to work in Chromium, which doesn't use this functionality yet. The work on
+  upstreaming the removal of static initializers is in progress:
+  https://github.com/google/protobuf/issues/1404
+
+- 0004-fix-integer-types-and-shared-library-exports.patch
+
+  This patch makes protobuf int64 to be int64_t (as opposed to long long in
+  upstream), and similarly for other integer types. It also allows exporting
+  protobuf symbols in Linux .so libraries, so that protobuf can be built as a
+  component (see http://crrev.com/179806).
+
+- 0005-fix-no-threadlocal-on-mac.patch
+
+  Apple's Clang doesn't support thread-local variables on Mac.
+
+- 0006-fix-clang-type-errors.patch
+
+  New versions of Clang have type checking semantics that check the conformance
+  with the C++ standard more thorougly, which makes them a bit more
+  restrictive. This cherry-picks upstream commits
+  3cc35adb6dca9057a790d253b9f5e2b9b000a106 and
+  58f07644eab25f68973ecab4bdc43fc318c26131.
+
+- 0007-uninline_googleonce.patch
+- 0008-uninline_get_empty_string.patch
+- 0009-uninline-arenastring.patch
+- 0010-uninline-generated-code.patch
+
+  These patches uninline some functions, resulting in a significant reduction
+  (somewhere between 500 KB and 1 MB) of binary size.