Bug 1471754 introduced "LegacyImplementation" for each target type.
This is legacy as it depends on the existing listProcesses, listRemoteFrames, listWorkers methods and processListChanged, workerListChanged events.
This is considered legacy as these methods do not match frontend needs and force to fetch the full list of all the targets everytime one is created or destroyed.

Instead, the actors should expose an API that match the TargetList API:

• startListening or watchTargets in order to declare that we want to receive all the targets of a given type. This will automatically emit all already existing targets. And then emit an event everytime there is a new target of this type being created or destroyed.
• stopListening or unwatchTargets, which unregister the listener and stop emitting creation or destruction events.

This work should most liekly be done by keeping the LegacyImplementations in order to support backward compatibility.

Tracking for Fission Nightly (M6)

This is going to be implemented in bug 1593937.