It's widely documented on the web that Apple enforces quarantine attributes
'for us' using the list in
/System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/Exceptions.plist
Unfortunately, this is based on the bundle ID. Which means it won't apply to custom
builds, to Nightly, and potentially to other cases. It would also be much nicer
if we could just make these determinations ourselves. Step 1: opt-in from Info.plist
Without this, for e.g. local Nightly builds, asking for quarantine attributes
simply gets you a nil dictionary and our code bails out.
MozReview-Commit-ID: 4aLyz2901BS
9486e1f8a4