Bug 826621 - when we clear a media frame, remember that we cleared it r=daleharvey a=blocking+

At some point relatively recently I started clearing media frames when leaving fullscreen mode so we don't keep decoded images around in memory. And I started setting the photo's filename on the frame object so  that I would know whether I needed to reload the photo.

But I didn't always delete the filename when I cleared the frame, so bug 826621 happens when I go back to an empty frame, but the frame thinks it already has the photo I want in it and I get a blank gray screen.