update test for whether the page is an image

The add-on stopped working in Fx 25 because of Bug 885177 - "Make
window.ImageDocument ChromeOnly". This changes the test to check for the
existence of the property imageResizingEnabled on the document object.

Two other properties would also have sufficed, "imageIsOverflowing", and "imageIsResized". Another approach that occurred to me was checking whether the "contentType" property began with "image/"