add back iframe (without src) support

[patricklx]

this makes inspector work again for iframes cases like https://ember-twiddle.com/

some time chrome introduced match_origin_as_fallback, which needs to be enabled for iframes (with srcdoc) to work.
and <iframe srcdoc="..."> will have the origin about:srcdoc

"match_origin_as_fallback" - Boolean
Optional. Defaults to false. Whether the script should inject in frames that were created by a matching origin, but whose URL or origin may not directly match the pattern. These include frames with different schemes, such as about:, data:, blob:, and filesystem:.