Bugfix: `match_origin_as_fallback` failing to fix content script injection into iframes with `sandbox` and `srcdoc` attrs

[mnholtz]

Follow-up to https://github.com/pixiebrix/pixiebrix-extension/pull/8922

We've identified setting match_origin_as_fallback in the manifest as a preferred approach to fixing https://github.com/pixiebrix/pixiebrix-extension/issues/8921

But neither @fungairino nor @mnholtz were able to replicate a fix locally with the PixieBrix Extension, despite being able to reproduce the fix with a simple reproduction detailed in https://issues.chromium.org/issues/355256366

This implies that there must be something specific to the new version of Chrome and the PixieBrix extension that is preventing the content script injection with these sort of iframes <iframe srcdoc="..." sandbox/>