Which has an optmization if both size of the string being passed
through are FlyStrings, which actually ends up being the case
in some places during selector matching comparing attribute names.
Instead of maintaining more overloads of
Infra::is_ascii_case_insensitive_match, switch
everything over to equals_ignoring_ascii_case instead.
Used by chess.com, where it stores URLs to assets in CSS URL variables.
It then receives the value of them with getComputedStyle() and then
getPropertyValue(). With this, it trims off the url('') wrapper with a
simple slice(5, -2). Since we didn't preserve the opening quotation, it
would slice off the `h` in `https` instead of the quotation.
