Remove usage of sandbox.importFunction

A change in one of the recent Nightly builds had completely broken GM by throwing NS_ERROR_UNEXPECTED. After some investigation, it looks like sandbox.importFunction has been made obsolete almost a year ago and one should just use

`sandbox.doSomething = function() { ... };`

to import functions. See [MDN](https://developer.mozilla.org/en/docs/Components.utils.Sandbox#Importing_functions_or_objects_into_the_Sandbox) for details. I tested this patch on today's Nightly and it seems to work fine without any errors.