This will be used by the inspector, for showing style sheet contents.
Identifying a specific style sheet is a bit tricky. Depending on where
it came from, a style sheet may have a URL, it might be associated with
a DOM element, both, or neither. This varied information is wrapped in
a new StyleSheetIdentifier struct.
(cherry picked from commit 51a426cc0562d12a6e8f7d1d1a38f9ecac77072a)
