With Site Isolation, potentially any frame in a document can be in a remote process. Code that interacts with frames (such as DevTools) needs to be updated to treat each frame as potentially remote. To allow this code conversion to happen in parallel with the DOM work to actually move things to separate processes, we should expose message managers[1] for all frames, even those currently in the same process. [1]: https://searchfox.org/mozilla-central/rev/84cea84b12145d752e50ddca6be5462c38510e35/dom/base/nsFrameLoader.cpp#3115

We probably don't want to do this directly as-is, as this will have significant overhead, creating new compartments for every frame in a document. We'll probably take a different approach which does not require creating many Message Managers. Morphing this bug into the approach we're more likely to take.