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

third_party/WebKit/Source/bindings/core/v8/SerializedScriptValue.h

 /*
  * Copyright (C) 2009, 2010 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 62 matching lines @@
   // Version 8: File.lastModified in milliseconds (seconds-based in earlier
   //            versions.)
   // Version 9: Added Map and Set support.
   // [versions skipped]
   // Version 16: Separate versioning between V8 and Blink.
   // Version 17: Remove unnecessary byte swapping.
   //
   // The following versions cannot be used, in order to be able to
   // deserialize version 0 SSVs. The class implementation has details.
   // DO NOT USE: 35, 64, 68, 73, 78, 82, 83, 85, 91, 98, 102, 108, 123.
+  //
+  // 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 constexpr uint32_t kWireFormatVersion = 17;
 
   struct SerializeOptions {
     STACK_ALLOCATED();
     Transferables* transferables = nullptr;
     WebBlobInfoArray* blob_info = nullptr;
     bool write_wasm_to_stream = false;
   };
   static PassRefPtr<SerializedScriptValue> Serialize(v8::Isolate*,
                                                      v8::Local<v8::Value>,
@@ skipping 130 matching lines @@
       ExceptionState& exception_state) {
     return SerializedScriptValue::Serialize(
         isolate, value, SerializedScriptValue::SerializeOptions(),
         exception_state);
   }
 };
 
 }  // namespace blink
 
 #endif  // SerializedScriptValue_h