Once legacy support for `Inspector.activateExtraDomains` is no longer needed, Web Inspector doesn't need to hold on to anything not supported by the current `WI.DebuggableType` (and associated `WI.TargetType`s) for feature checking, as once the `WI.DebuggableType` is known it will not change (nor will the associated `WI.TargetType`s). Additionally, we could even streamline the process of ingesting `InspectorBackendCommands.js` by querying for the `WI.DebuggableType` (and associated `WI.TargetType`s) up front or even generate the data structure and model objects directly.