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

BUG=chromium:715232

Review-Url: https://codereview.chromium.org/2839903003
Cr-Commit-Position: refs/heads/master@{#44947}

src/value-serializer.cc

@@ -30,6 +30,14 @@ namespace internal {
 // 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_assert(kLatestVersion == v8::CurrentValueSerializerFormatVersion(),
               "Exported format version must match latest version.");