Don't show the abandon download dialog when closing normal profile window on Mac

On Mac, chrome is not killed after closing normal profile windows.
So there is no need to show the abandon download dialog as download
can still continue when all windows are closed. If user really kills
chrome through command line or system tray, he can resume the download
from downloads page later.

BUG=955044

Change-Id: I02d7205b2bce4a8c96018bc9c6cfcf797e1b4394
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1618062
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661412}
3 files changed