Bug 995880. Correctly include non-heap memory use by VolatileBuffer's in memory use of images. r=mwu

Bug 962670 added VolatileBuffer's but only included the heap memory use.
Bug 964187. When the frame for an image is destroyed remove it from the list of visible images. r=mats

This prevents us from holding onto the image but also doesn't create any more chance that it will be discarded if the frame comes back right away (ie a frame reconstruct, or moving it around in the DOM tree).