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

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

Index: third_party/WebKit/Source/bindings/core/v8/SerializedScriptValue.h
diff --git a/third_party/WebKit/Source/bindings/core/v8/SerializedScriptValue.h b/third_party/WebKit/Source/bindings/core/v8/SerializedScriptValue.h
index 15d211c929faab4a4aa82561f774dedc5a5c0a58..84019385b1ba682eb1a5691f1241355388142b2d 100644
--- a/third_party/WebKit/Source/bindings/core/v8/SerializedScriptValue.h
+++ b/third_party/WebKit/Source/bindings/core/v8/SerializedScriptValue.h
@@ -80,6 +80,12 @@ class CORE_EXPORT SerializedScriptValue
   // 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 backward-incompatible change.

[jsbell, 2017/04/25 20:08:38]

"backward-incompatible" isn't quite correct - "backwards compatibile" means new
code can read old data, which we always maintain.

I don't know a succinct way to say it; maybe "breaks forwards-compatibility for
previous releases" ?

[jbroman, 2017/04/25 20:13:43]

Ah, you're quite correct. Hmm...I'm not sure your wording is as clear as I'd
like. WDYT of:

  // 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.

+  // It is strongly recommended that you do not make backward-incompatible wire
+  // format changes if it is 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 {