To match the behavior of StyleSheetHandle so that we can simply replace
uses of StyleSheetHandle with StyleSheet* in later patch.
MozReview-Commit-ID: LfGKrUmzC4h
One of the annoying things about sharing algorithms on the superclass here is that
certain members live on StyleSheet and others live on StyleSheetInfo (a situation
resulting from the split between CSSStyleSheet and CSSStyleSheetInner). This allows
us to write general algorithms on StyleSheet that touch members on StyleSheetInfo
without paying the price of virtual dispatch.