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
 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.py,
+   protobuf_lite.gypi, protobuf_nacl.gyp).
+4. Add open-source library six.py to protobuf/third_party/six/six.py
+5. Apply patches in patches/:
 
-A protobuf.gyp file has been added for building with Chromium.
+   $ for patch in patches/*; do patch -s -p1 < $patch; done
 
-This code has been patched to support unknown field retention in protobuf-lite.
-See r62331 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.
 
-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.
+   $ cd $SRC_DIR
+   $ ninja -C out/Debug protoc
+   $ cd third_party/protobuf/src
+   $ ../../../out/Debug/protoc --python_out=../python google/protobuf/descriptor.proto
+   $ cd ..
 
-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.
-
-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
-
-GetEmptyString() and GoogleOnceInit() have been uninlined, for a large savings
-in binary size.
-
-A BUILD.gn file has been added for building with GN.
-
-Cherry-pick pherl changes to make protobuf build on VS2015.
-
-Cherry-pick c3cb53b (fix "sometimes-uninitialized" warning).
-
-Cherry-pick https://github.com/google/protobuf/commit/56a90a2081379a5
-
-Cherry-pick https://github.com/google/protobuf/commit/69d660b39ceabea
-
-Cherry-pick https://github.com/google/protobuf/commit/a74e912a8be1274
+   Copy the resulting descriptor_pb2.py to protobuf/.
+6. Add empty __init__.py to protobuf/.
+7. Update README.chromium.