Handle iframes with srcdoc

[Treora]

We currently do not handle content of an <iframe srcdoc="....">. We inline the iframe's inner doc, when available, into the src as a data: URL, regardless whether it came from the src or from a srcdoc; but a viewer will use the srcdoc instead of src when it is available, thus things break.

We could choose to always remove srcdoc attributes (or always use them), or we could process srcdoc content as a subresource (except we don't even need to fetch it, and its base URL is equal to the parent document's base URL) and keep it in the srcdoc attribute. Or something in between.

[Treora]

Similarly, a (i)frame without either src or srcdoc is ignored. The frame may nevertheless have content inserted into it by a script (e.g. Etherpad content is currently not stored because of this issue). Time to start seeing a frame as more than just a single subresource..

[Treora]

Another note: a frame with e.g. an image as its src will, both in Firefox and Chromium (but each slightly differently), be rendered into an ad-hoc generated DOM. We currently snapshot that DOM, but should just inline the image file instead.