Before, `about: srcdoc` was only handled when loading `<iframe>`
contents in the same `ScriptThread` as the parent. This is not always
the case though with sandboxed `<iframe>`s. This change makes it so that
both code paths properly handle `about: srcdoc`.
Testing: This causes around 12 new WPT passes.
Fixes: #36529.
Fixes: #27791.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>