Add configurable sort mode for the source list

The order of the source list is sometimes changing seemingly at random.

This CL adds an additional sort mode that uses the order that is
returned from CGWindowListCopyWindowInfo().

The sort mode is configurable through the Finch feature
ThumbnailCapturerMac sort_mode. The following values are
valid for sort_mode

kNone - No extra sorting, same order as returned by SCShareableContent.
kCGWindowList - Same order as returned by CGWindowListCopyWindowInfo().
kNewWIndowsLast - Static order where new windows are put last in the
                  list.

Bug: chromium:1480164
Change-Id: Idfbadcb9fd9cfeb29598a4d777751a46081d17fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4847753
Reviewed-by: Elad Alon <eladalon@chromium.org>
Commit-Queue: Johannes Kron <kron@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1195707}