Revert "third_party/protobuf: Update to HEAD (83d681ee2c)"

third_party/protobuf/objectivec/Tests/unittest_cycle.proto

 // Protocol Buffers - Google's data interchange format
 // Copyright 2015 Google Inc.  All rights reserved.
 //
 // 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.
 //     * Redistributions in binary form must reproduce the above
@@ skipping 13 matching lines @@
 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 syntax = "proto2";
 
 package protobuf_unittest;
 
-// Cycles in the Message graph can cause problems for message class
-// initialization order.
+// 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.
 
 // You can't make a object graph that spans files, so this can only be done
 // within a single proto file.
 
 message CycleFoo {
   optional CycleFoo a_foo = 1;
   optional CycleBar a_bar = 2;
   optional CycleBaz a_baz = 3;
 }
 
 message CycleBar {
   optional CycleBar a_bar = 1;
   optional CycleBaz a_baz = 2;
   optional CycleFoo a_foo = 3;
 }
 
 message CycleBaz {
   optional CycleBaz a_baz = 1;
   optional CycleFoo a_foo = 2;
   optional CycleBar a_bar = 3;
 }