Bump vendoring, move to proto3 release.

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

Index: recipe_engine/third_party/google/protobuf/internal/containers.py
diff --git a/recipe_engine/third_party/google/protobuf/internal/containers.py b/recipe_engine/third_party/google/protobuf/internal/containers.py
index 97cdd848e34d8da4083abb512b7403686ddec91f..ce46d08c9eca71ccddac3df8c7a6958d40e4505c 100644
--- a/recipe_engine/third_party/google/protobuf/internal/containers.py
+++ b/recipe_engine/third_party/google/protobuf/internal/containers.py
@@ -594,7 +594,11 @@ class MessageMap(MutableMapping):
 
   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.