I think the only change is that we now support appending of values (so we don't get holes).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/138fde45864d92ce1315f7e0d8c907230c2ffa68

commit 138fde45864d92ce1315f7e0d8c907230c2ffa68
Author: Darren Shen <shend@chromium.org>
Date: Mon Feb 19 23:23:48 2018

[css-typed-om] Allow mutations of CSSUnparsedValue.

This patch allows CSSUnparsedValue items to be mutated/appended.
See https://github.com/w3c/css-houdini-drafts/issues/664

Bug:  812919 
Change-Id: I3e3c1d91fa9bfaa3a1a4adde45202c36c9bdb37f
Reviewed-on: https://chromium-review.googlesource.com/923670
Reviewed-by: nainar <nainar@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537691}
[modify] https://crrev.com/138fde45864d92ce1315f7e0d8c907230c2ffa68/third_party/WebKit/LayoutTests/external/wpt/css/css-typed-om/stylevalue-subclasses/cssUnparsedValue.html
[modify] https://crrev.com/138fde45864d92ce1315f7e0d8c907230c2ffa68/third_party/WebKit/Source/core/css/cssom/CSSUnparsedValue.cpp
[modify] https://crrev.com/138fde45864d92ce1315f7e0d8c907230c2ffa68/third_party/WebKit/Source/core/css/cssom/CSSUnparsedValue.h
[modify] https://crrev.com/138fde45864d92ce1315f7e0d8c907230c2ffa68/third_party/WebKit/Source/core/css/cssom/CSSUnparsedValue.idl