Bug 1056340 - Clip viewport fixed position overflow to scroll frame. r=roc
Bug 1063859 - Do not iterate on undefined valued when querying notifications. r=mhenretty

When we are performing the getAll/getAllCrossOrigin requests and there
is no notifications, then this.notifications[origin] (with a valid
origin) is just undefined. We should make sure we do not iterate over in
this case.