Layout Test css-custom-properties-api/registerProperty.html is failing

[Per Arne Vollan]

The following layout test is failing Windows:

css-custom-properties-api/registerProperty.html

Probable cause:

Unknown.

Flakiness Dashboard:

https://webkit-test-results.webkit.org/dashboards/flakiness_dashboard.html#showAllRuns=true&tests=css-custom-properties-api%2FregisterProperty.html

[Per Arne Vollan]

Updated test expectations in <https://trac.webkit.org/changeset/236318/webkit>.

[Karl Dubost]

Currently the message fails with 

```
FAIL message: CSS.registerProperty is not a function. (In 'CSS.registerProperty({
    name: name,
    syntax: syntax,
    initialValue: initial,
    inherits: inherits
  })', 'CSS.registerProperty' is undefined)
```

The associated Firefox bug is
https://bugzilla.mozilla.org/show_bug.cgi?id=1634362
which has a dependency on Houdini.
https://bugzilla.mozilla.org/show_bug.cgi?id=1273706

see https://developer.mozilla.org/en-US/docs/Web/API/CSS/registerProperty
and https://drafts.css-houdini.org/css-properties-values-api/#the-registerproperty-function

[Antti Koivisto]

This is done.

[Radar WebKit Bug Importer]

<rdar://problem/104478612>