It would help to easily access to the related SessionData and/or WatcherActor's context from the target actor (and any of its children/resource watcher).
It would help for example know if we are in a browser toolbox context, or easily retrieve watcher's traits, ...

https://phabricator.services.mozilla.com/D128897?id=498275#inline-709043
https://phabricator.services.mozilla.com/D128897?id=498275#inline-709060

This helps better differentiates cases where we instantiate a target actor
with a watcher actor and possibly debug the whole browser and instantiate many targets.
Or when we instantiate only one target actor, like when we debug workers or xpcshell from about:debugging.
For now, in this case, there is no watcher actor involved and we manually instantiate the target from the descriptor.

https://hg.mozilla.org/mozilla-central/rev/87afa7acff1a