Pull new version of protobuf sources.

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
+License File: LICENSE
 Version: unknown
-Revision: r476
+Revision: 6994b07f1480f658e2ea6262f8ca804e836ef1d2
 Security Critical: yes
 
-Local files (not taken from upstream):
-README.chromium
-config.h
-descriptor2_pb.py
+Steps used to create the current version:
+(1) Download and unpack protobuf sources from
+    https://github.com/google/protobuf/archive/master.zip .
+(2) Apply the patches in patches/ in order.  In all these cases, after
+    determining how to adapt the patch to the current protobuf codebase, you
+    should regenerate the patch for the next maintainer.
 
-A protobuf.gyp file has been added for building with Chromium.
-
-This code has been patched to support unknown field retention in protobuf-lite.
-See r62331 for the patch.
-
-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.
-
-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
+    A brief description of each patch:
+  00: Add initial GYP/GN buildfiles, DEPS, OWNERS, and README.chromium.  You
+      will need to modify the README to include the correct revision, and modify
+      the buildfiles to reflect the actual files in the source tree, what they
+      #include, what warnings they trigger, etc.
+  01: Miscellaneous build fixes to make the upstream sources compile.  At this
+      point you should be able to build the protobuf_* and protoc targets (but
+      not necessarily anything depending on them).
+  02: Add unknown field retention to protobuf_lite.  This is a large and complex
+      patch which will likely require reworking to apply correctly.  Based on
+      http://crrev.com/62331 and http://crrev.com/173228 .
+  03: Convert protobuf_lite to a component.  Based on http://crrev.com/179806 .
+  04: Uninline various functions.  Based on http://crrev.com/307332 .
+(3) 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.
+  $ cd $SRC_DIR
+  $ ninja -C out/Debug protoc
+  $ cd third_party/protobuf/src
+  $ ../../../out/Debug/protoc --python_out=../python google/protobuf/descriptor.proto
+  $ cd ..
+  $ git add -f python/google/protobuf/descriptor_pb2.py
+  $ git commit
+(4) At this point you should be done, and able to successfully build everything.