Bug 200854

After speaking at length offline with Joe about what `enable`/`disable` represent, we've come to the following conclusion: An agent's `enable`/`disable` controls whether that agent is "allowed" to send events to the connected frontend. For now, an agent isn't "required" to be `enable`d in order for commands being sent from the frontend to the backend to have any effect (some agents already work this way). Agents that need other agents to be `enable`d shouldn't ever assume anything about the state of the other agent, and therefore should properly handle the case where that other agent isn't enabled (often this is just returning an error from the command or not sending an event). As such, we should: - add `enable`/`disable` to all domains that have events - re-add them to the `Page` domain (partial "undo" of r248454) - this does NOT include the `DOM` domain, as that's going to be handled by <https://webkit.org/b/197025> - add error messages when `enable` is called if the domain is already enabled - add proper checks when agents are referenced by other agents I think we should move towards a world where `enable`/`disable` is the single entry/exit point for ALL agents, but that can come later.