Note that as discussed on IRC, this makes the viewer slightly slower to load *only* in `gulp server` mode, however the difference seem slight enough that I think it will be fine.
Furthermore we introduce two new methods named `setCallback` and
`setReason` so external code does not change the properties of the class
directly. Finally we update various names of properties and methods to
be more self-explanatory.
Fixes 6898.
Note that this doesn't prevent the warning for PDF files that *do* ask for a password, e.g. http://async5.org/moz/passwordOU.pdf, but it's not clear to me if/how we could avoid that.