Bump vendoring, move to proto3 release.

recipe_engine/third_party/google/protobuf/internal/containers.py

 # Protocol Buffers - Google's data interchange format
 # Copyright 2008 Google Inc.  All rights reserved.
 # https://developers.google.com/protocol-buffers/
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
@@ skipping 576 matching lines @@
     return len(self._values)
 
   def __iter__(self):
     return iter(self._values)
 
   def __repr__(self):
     return repr(self._values)
 
   def MergeFrom(self, other):
     for key in other:
-      self[key].MergeFrom(other[key])
+      # According to documentation: "When parsing from the wire or when merging,
+      # if there are duplicate map keys the last key seen is used".
+      if key in self:
+        del self[key]
+      self[key].CopyFrom(other[key])
     # self._message_listener.Modified() not required here, because
     # mutations to submessages already propagate.
 
   def InvalidateIterators(self):
     # It appears that the only way to reliably invalidate iterators to
     # self._values is to ensure that its size changes.
     original = self._values
     self._values = original.copy()
     original[None] = None
 
   # This is defined in the abstract base, but we can do it much more cheaply.
   def clear(self):
     self._values.clear()
     self._message_listener.Modified()