[Mac] Fix crash when opening new tab in forced incognito mode

The existing code was requesting a regular (non-incognito) browser,
triggering a CHECK in CreateBrowser:
https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/app_controller_mac.mm;drc=75b79876557f7ad6e1b1fdc9cbb45dac842d213a;l=164

Fixed: 1206726
Change-Id: I0598989539ac3cb91ad9a2634e8632f18a32d0ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2880001


Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#880994}