[upgrade_detector] Avoid the macOS Catalina file consent dialog.

This CL switches to using a "trivial" watch on macOS, whereby *only* the
given directory is watched for changes. In particular, parent
directories are not also monitored. It is this monitoring of parent dirs
that may trigger the consent dialog.

BUG=1147071

Change-Id: Ibbacce44ea838082aa4e92e865033ecdec0f3689
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2546715
Commit-Queue: Greg Thompson <grt@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830353}