This patch modifies SessionStoreInternal.onMoveToNewWindow to raise a more specific error message when
no TabState is found for the tab being moved, and then allows that more specific error message
as an uncaught rejection in browser_ext_tabs_events.js through PromiseTestUtils.allowMatchingRejectionsGlobally.
These changes don't address the underlying issue hit by SessionStoreInternal, but in the short term
it will allow to prevent the high frequency intermittent failure currently hit by the
browser_ext_tabs_events.js (and to keep the existing test coverage which would be lost if we
would instead skip the entire test file to workaround the intermittency).
An inline comment added right above the call to PromiseTestUtils.allowMatchingRejectionsGlobally
includes a mention to Bug 1938594 (a new bug filed under the "Firefox :: Session Restore" bugzilla
component to track additional investigations on the SessionStore/TabStateFlusher side) as a reminder
of the underlying issue (and to remove the workaround when it may not be necessary anymore).
Differential Revision: https://phabricator.services.mozilla.com/D230528
775328ed67