scrollbars.css is the only sheet which is parsed as author level, but later
added as agent level in [1]. Add a new enum value so that it can be parsed
as author level in gecko (nsCSSParser::AgentRulesEnabled() will exclude it),
but servo can recognize it as agent level sheet when the sheet is created.
Delete UserRulesEnabled() because no one uses it.
[1] http://searchfox.org/mozilla-central/rev/7419b368156a6efa24777b21b0e5706be89a9c2f/layout/base/nsDocumentViewer.cpp#2326
MozReview-Commit-ID: 2lrV4ogfnHM
