URI the sheet was loaded from, its certificate (if any) and the loader
principal, not just on the page the sheet is being applied to. Propagate this
principal as the loading principal to @import loads from stylesheets. Change
sheet access checks to use this principal instead of the sheet URI. Bug
221428, r+sr=peterv
git-svn-id: svn://10.0.0.236/trunk@225401 18797224-902f-48f8-a5cc-f745e15eee43