Update and fix icons on the Mac

The Dock icon drawing code was using a path-based API, and those are
getting phased out, so move it to a different API. Add comments to
clarify the choice, and update the formatting in the file while here.

In the intent picker helper, the intention was to get a "generic
app icon" in case of failure, but the "current app icon" constant was
mistakenly used instead. Use the correct, undocumented, string value.

Bug: none
Change-Id: Ieed78a4b9540bb97d28d30cafd0915322884580f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5598438
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Auto-Submit: Avi Drissman <avi@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Reviewed-by: Leonard Grey <lgrey@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1310228}