[PWAs] Move what is remaining of web_app_shortcut_mac.mm to mac folder.

This is part 9/x of a series of CLs to clean up/organize the mac
PWA OS integration code. This CL merely moves the now small remainder
of web_app_shortcut_mac.mm to the mac/ subdirectory, just to keep all
mac code together.

Bug: 347167972
Change-Id: Ic162adf52657f549fff4c11c59dd28752a619def
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5631238
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1318638}