third_party/protobuf: Update to HEAD (83d681ee2c)

third_party/protobuf/objectivec/Tests/unittest_cycle.proto

Index: third_party/protobuf/objectivec/Tests/unittest_cycle.proto
diff --git a/third_party/protobuf/objectivec/Tests/unittest_cycle.proto b/third_party/protobuf/objectivec/Tests/unittest_cycle.proto
index 5f6f56a1aee1d91b3db2cc2b5cc45192734596bc..afc1b0fe275d2e5f527dd87320185cf24808a92d 100644
--- a/third_party/protobuf/objectivec/Tests/unittest_cycle.proto
+++ b/third_party/protobuf/objectivec/Tests/unittest_cycle.proto
@@ -31,10 +31,8 @@ syntax = "proto2";
 
 package protobuf_unittest;
 
-// Cycles in the Message graph can cause problems for the mutable classes
-// since the properties on the mutable class change types. This file just
-// needs to generate source, and that source must compile, to ensure the
-// generated source works for this sort of case.
+// Cycles in the Message graph can cause problems for message class
+// initialization order.
 
 // You can't make a object graph that spans files, so this can only be done
 // within a single proto file.