Replay: Don't capture iframe if it relies on `srcdoc` instead of `src`

[bruno-garcia]

Since rrweb doesn't mask the contents of the iframe when it relies on srcdoc. It's best we simply default to blocking it.

[chargome]

@bruno-garcia is this still an issue? Just tested it with a react app and everything was masked there.

[chargome]

Closing since we can't reproduce it

The issue is that the content of srcdoc is still visible when inspecting the dom in the replay.