[sheriff] Mark AcceptValidShareTarget_PayloadSuccessful as flaky

This test causes consistently build failure.

TBR = himanshujaju@chromium.org
TBR = alexchau@chromium.org

Bug: 1123022
Change-Id: I9e8270ac73118ad6f571374125f9c55411a7ab9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2381067


Commit-Queue: Tina Wang <tinazwang@chromium.org>
Reviewed-by: Tina Wang <tinazwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802679}