Make custom properties that are IACVT guaranteed-invalid

Per recent spec change [1], custom properties that are invalid at
computed-value time (IACVT) shall become guaranteed-invalid [2]
if the custom property supports guaranteed-invalid values.

[1] https://github.com/w3c/csswg-drafts/pull/6006
[2] https://drafts.csswg.org/css-variables/#guaranteed-invalid-value

Fixed: 1110188
I2S: https://groups.google.com/a/chromium.org/g/blink-dev/c/0xrbzYe_vxU/m/7bsL76n9CgAJ
Change-Id: Ibfbb0c641abaf29f099e082115d5f04f77637840
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2734621


Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#860161}