[Mac Browser Tests] Prevent Finder window storm during tests

Certain operations create Finder windows, but when those operations
occur during tests, the result is a flood of Finder windows on the bots.
This cl prevents another avenue of Finder window creation during tests
(the first was crrev.com/c/3449586, which this cl was patterned after).

Bug: 1417591

Test: out/Default/browser_tests \
    --gtest_filter=DownloadExtensionTest.DownloadsShowFunction

Change-Id: Icc30e36093953664311c4935c71ae03e20a54564
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4394288


Commit-Queue: Jayson Adams <shrike@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148020}