Add fieldtrial entry for BackgroundCrxDownloaderMac

We will be doing a dogfood experiment of this feature on Canary. A fieldtrial
entry is required for testing.

BUG=1493709

Change-Id: I5dbc2468bc374658558861e62151176466c896ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5002248


Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Commit-Queue: Noah Rose Ledesma <noahrose@google.com>
Cr-Commit-Position: refs/heads/main@{#1218977}