[Discover MultiColumn] Added feature flag

Bug: 1360953
Change-Id: Idbee5ca01cb5d6696b90bd108448704fc66cf753
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3879667


Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Reviewed-by: Neil Coronado <nemco@google.com>
Commit-Queue: Sirisha Kavuluru <skavuluru@google.com>
Cr-Commit-Position: refs/heads/main@{#1044787}