[css-values-4][css-images-4] Style resource fetching should probably use credential mode "include" in the default case

[weinig]

I was talking with @annevk, and he mentioned that he thought the "fetch a style resource" algorithm in CSS Values 4 should probably follow HTML (and at least what WebKit does) and use credentials mode is "include" rather than "same-origin" as the default value.

This would better match HTML's "create a potential-CORS request" algorithm used for most HTML Subresource fetching.

(@annevk, please correct anything I have wrong here).

cc @tabatkins, @fantasai

[annevk]

cc @noamr