Calling IsLink ignores whatever state we have for aTreeMatchContext and will trigger an assertion if aTreeMatchContext.mForStyling is false. This also adds a comment about what that line effectively does.
This should allow pseudo-classes that accept selector lists to properly handle complex selectors. `:visited` is also matched inside selector lists.
A previous iteration of this commit had incorrect matching behavior when called from `RestrictedSelectorListMatches`.
This makes it clear that we're accessing a selector list (nsCSSSelectorList), which is different from the selectors (nsCSSSelector) contained inside a selector list. Previously, both were confusingly referred to as mSelectors.
This modifies selector list parsing to accommodate being "forgiving". Aliases for the :is selector's former names were also included. Note that the older and prefixed variant `-moz-any` remains unforgiving.
