ValueSerializer: Add a warning about making wire format changes near branch.

src/value-serializer.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/value-serializer.h"
 
 #include <type_traits>
 
 #include "src/base/logging.h"
 #include "src/conversions.h"
@@ skipping 11 matching lines @@
 
 namespace v8 {
 namespace internal {
 
 // Version 9: (imported from Blink)
 // Version 10: one-byte (Latin-1) strings
 // Version 11: properly separate undefined from the hole in arrays
 // Version 12: regexp and string objects share normal string encoding
 // Version 13: host objects have an explicit tag (rather than handling all
 //             unknown tags)
+//
+// WARNING: Increasing this value is a change which cannot safely be rolled
+// back without breaking compatibility with data stored on disk. It is
+// strongly recommended that you do not make such changes near a release
+// milestone branch point.
+//
+// Recent changes are routinely reverted in preparation for branch, and this
+// has been the cause of at least one bug in the past.
 static const uint32_t kLatestVersion = 13;
 
 static const int kPretenureThreshold = 100 * KB;
 
 template <typename T>
 static size_t BytesNeededForVarint(T value) {
   static_assert(std::is_integral<T>::value && std::is_unsigned<T>::value,
                 "Only unsigned integer types can be written as varints.");
   size_t result = 0;
   do {
@@ skipping 1978 matching lines @@
   if (stack.size() != 1) {
     isolate_->Throw(*isolate_->factory()->NewError(
         MessageTemplate::kDataCloneDeserializationError));
     return MaybeHandle<Object>();
   }
   return scope.CloseAndEscape(stack[0]);
 }
 
 }  // namespace internal
 }  // namespace v8