Previously the WebProgressListener in AboutWelcomeChild was detecting
OnLocationChange notifications for loading about:welcome rather than
notifications for navigating away from it, meaning that
AWTerminate.UNKNOWN was almost immediately replaced as soon as the
document was loaded, rather than only when navigating away. The
DOMDocElementInserted event fires slightly later than DOMWindowCreated
so the actor no longer sees the OnLocationChange notification for the
current document, causing tests to fail.
As this telemetry is no longer actively monitored, this patch removes
AWTerminate.UNKNOWN to preserve the existing behaviour.
Differential Revision: https://phabricator.services.mozilla.com/D118620
938c67f546