Bug 1033679. When we clear frames from the image loader make sure that we also remove the images from the refresh driver. r=khuey

Normally a frame being destroyed would do this via PresShell::NotifyDestroyingFrame -> ImageLoader::DropRequestsForFrame but when we are tearing down the whole frame tree we skip per frame notifications like that for perf reasons. So we now make sure to do it when clearing out frames wholesale.

We also need to pass a prescontext pointer to ClearFrames because the ImageLoader only has a document pointer, and by that point in the presshell destruction sequence the presshell pointer on the document has been cleared.