Handle response images in 'avoidscailingimaqges' advice.

[tobli]

If avoidscailingimaqges advice can report incorrect image urls for img tags with srcset, but no src. http://www.aftonbladet.se/ is one such size (at least at the time of reporting). Since no src is present, the url get's reported as http://www.aftonbladet.se/.

Finding candidate images can be done with the following snippet:
Array.prototype.slice.call(document.getElementsByTagName('img')).filter((img) => img.src === '')

The coach needs to consider srcset, and update advice accordingly.