On Windows with high-DPI displays (150%+ scaling), Chromium
re-evaluates zoom on focus change (alt-tab). The persisted zoom
level was only re-asserted on show/restore/resize/move events.
Add 'focus' to zoomReassertWindowEvents() so the zoom level is
restored when the window regains keyboard focus, covering both
main and secondary session windows through the existing
installZoomReassertOnWindowEvents() abstraction.
Extend zoom.test.ts to exercise the registered focus handler.
Fixes#50837
QLYF
253d4aa996