The change in CustomizableUI is required because the routine wasn't designed to handle
custom elements with shadow roots. The routine's job is to walk up the ancestry of
the click event and determine if any of the parents have certain characteristics
that would indicate that the event should not close the panel, and without this
change it would bail out as soon as it hit the shadow root if the event's original
target was inside the custom element.
In this case, the element enclosing the fxa-menu-message is the "appMenu-fxa-menu-message"
toolbaritem with `closemenu="none"`, which this modification now lets
CustomizableUI detect.
Differential Revision: https://phabricator.services.mozilla.com/D230560
e06d4179af