Fix DMG installation code

In January 2023, https://crrev.com/c/4127122 did "cleanup" of the DMG
installation code, but broke it by accidentally inverting a
precondition check.

This wasn't noticed with the macOS 14 release when this code is
usually checked, but this was noticed with the macOS 15 check.

This CL unbreaks it by fixing the precondition check. This has been
verified by building a release build of Chromium and putting it onto
a disk image, and making sure it correctly shows the dialog with
macOS 15 in a VM.

Fixed: 346495941
Change-Id: I104bd853d9060c29aca0ec85026ecf07ec714882
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5621925
Commit-Queue: Mark Mentovai <mark@chromium.org>
Auto-Submit: Avi Drissman <avi@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313763}