Revert https://codereview.chromium.org/1291903002 (protobuf roll).

third_party/protobuf/README.chromium

 Name: Protocol Buffers
 Short Name: protobuf
-URL: https://github.com/google/protobuf
+URL: http://protobuf.googlecode.com/svn/trunk
 License: BSD
-License File: LICENSE
+License File: COPYING.txt
 Version: unknown
-Revision: 6994b07f1480f658e2ea6262f8ca804e836ef1d2
+Revision: r476
 Security Critical: yes
 
-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.
+Local files (not taken from upstream):
+README.chromium
+config.h
+descriptor2_pb.py
 
-    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.  Changes to
-      .pb.* files can likely be autogenerated once you apply the rest of the
-      patch and rebuild protoc; look at step (3) below and the
-      generate_descriptor_proto.sh script in the protobuf root directory for a
-      guide.  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.
+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