DNR not applied to fetch calls initiated in extension pages

When extensions do fetch calls to a third party domain, both WebKit and Orion do NOT modify the requests.

See demo extension:
https://jeurissen.co/webext-demos/dnr-in-extension-page

And WebKit bug:
https://bugs.webkit.org/show_bug.cgi?id=315369

Apple Radar:
rdar://problem/177727052

And orion bug:
https://orionfeedback.org/d/13891-dnr-does-not-modify-fetch-requests-initiated-from-extension-pages

Start by reproducing the behavior with the linked demo extension and then read the linked WebKit and Orion bug reports. The issue names no repository files or tests; completion would require confirming whether the external browser behavior is fixed and documenting the result.

Written by the indexing model from the issue text.