[cssom-1] Should `CSSStyleSheet.href` be `null` for a constructed style sheet?

[cdoublev]

The spec wants it to be "the base URL of the associated Document for the current global object", but it is currently null in Chrome and Firefox.

• https://drafts.csswg.org/cssom-1/#dom-stylesheet-href
• https://drafts.csswg.org/cssom-1/#create-a-constructed-cssstylesheet