Update protobuf to r428, part 1.

third_party/protobuf/python/google/protobuf/internal/decoder.py

Index: third_party/protobuf/python/google/protobuf/internal/decoder.py
===================================================================
--- third_party/protobuf/python/google/protobuf/internal/decoder.py	(revision 216642)
+++ third_party/protobuf/python/google/protobuf/internal/decoder.py	(working copy)
@@ -576,6 +576,7 @@
   local_SkipField = SkipField
 
   def DecodeItem(buffer, pos, end, message, field_dict):
+    message_set_item_start = pos
     type_id = -1
     message_start = -1
     message_end = -1
@@ -614,6 +615,11 @@
         # The only reason _InternalParse would return early is if it encountered
         # an end-group tag.
         raise _DecodeError('Unexpected end-group tag.')
+    else:
+      if not message._unknown_fields:
+        message._unknown_fields = []
+      message._unknown_fields.append((MESSAGE_SET_ITEM_TAG,
+                                      buffer[message_set_item_start:pos]))
 
     return pos