[css-properties-values-api] Disable CSS.registerProperty.

It now looks as if the spec may need to change in a way that's
very much backwards-incompatible (CSS.supports behavior could
change) [1].

It definitely sucks, but releasing this now is too risky.

[1] https://github.com/w3c/css-houdini-drafts/issues/902

Bug: 641877
Change-Id: I321a112255d91b815ac050a242707dc12f184e4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1670208
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671249}