- Make registerTraceableChannel calls synchronous. - Drop remoteTab parameter from registerTraceableChannel; validate process access based on remoteType checks instead. - In practice a null remoteTab was only supported for in-process extensions, but this was not obvious nor enforced. Enforce it too, and fix test that depended on null remoteTab. Differential Revision: https://phabricator.services.mozilla.com/D304595