Note that CSSOMString = DOMString is perfectly compliant with the spec.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/22f7bd8b0541ed47a6e906c1d79d12f7eb86660f

commit 22f7bd8b0541ed47a6e906c1d79d12f7eb86660f
Author: Hwanseung Lee <hs1217.lee@samsung.com>
Date: Tue May 08 11:40:16 2018

[css-typed-om] Use CSSOMString instead of DOMString

We should switch from DOMString to CSSOMString as
resolved in CSSWG resolved to use CSSOMStrings[1].

and there is idl issue when union type
have set of typedef value.
so this case did not change and will follow up
(https://crbug.com/838890)

[1]https://github.com/w3c/css-houdini-drafts/issues/687#issuecomment-379669504

Bug: 834164
Change-Id: Id4768a6edbcdc17bfa72cffd4a63c81d8124d256
Reviewed-on: https://chromium-review.googlesource.com/1034618
Commit-Queue: Hwanseung Lee <hs1217.lee@samsung.com>
Reviewed-by: Darren Shen <shend@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556760}
[modify] https://crrev.com/22f7bd8b0541ed47a6e906c1d79d12f7eb86660f/third_party/blink/renderer/core/core_idl_files.gni
[modify] https://crrev.com/22f7bd8b0541ed47a6e906c1d79d12f7eb86660f/third_party/blink/renderer/core/css/cssom/css_keyword_value.idl
[modify] https://crrev.com/22f7bd8b0541ed47a6e906c1d79d12f7eb86660f/third_party/blink/renderer/core/css/cssom/css_numeric_value.idl
[modify] https://crrev.com/22f7bd8b0541ed47a6e906c1d79d12f7eb86660f/third_party/blink/renderer/core/css/cssom/css_style_value.idl
[modify] https://crrev.com/22f7bd8b0541ed47a6e906c1d79d12f7eb86660f/third_party/blink/renderer/core/css/cssom/css_unit_value.idl
[modify] https://crrev.com/22f7bd8b0541ed47a6e906c1d79d12f7eb86660f/third_party/blink/renderer/core/css/cssom/css_unparsed_value.idl
[modify] https://crrev.com/22f7bd8b0541ed47a6e906c1d79d12f7eb86660f/third_party/blink/renderer/core/css/cssom/css_variable_reference_value.idl
[modify] https://crrev.com/22f7bd8b0541ed47a6e906c1d79d12f7eb86660f/third_party/blink/renderer/core/css/cssom/style_property_map.idl
[modify] https://crrev.com/22f7bd8b0541ed47a6e906c1d79d12f7eb86660f/third_party/blink/renderer/core/css/cssom/style_property_map_read_only.idl
[add] https://crrev.com/22f7bd8b0541ed47a6e906c1d79d12f7eb86660f/third_party/blink/renderer/core/css/cssom_string.idl