Introduces a new kind of target "main-process",
which is not bound to any particular browser-element/browsing-context.
This implements only a subtest of domains and typical won't support the "content" ones.
This will require some more work in order to acknowledge having multiple kinds of targets.
We will also have "worker", "addon" and remoted-frames which will be very similar to tabs.
Depends on D22690
Differential Revision: https://phabricator.services.mozilla.com/D22691
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extra : moz-landing-system : lando
Rather than spinning up a separate WebSocket listener for each target, this
enables us to serve WebSocket connections from the original HTTPD that we serve
the rest of the JSON API from.
Victor Porof
Alexandre Poirot
Andreas Tolfsen