Use api.tryserver.gerrit_change_target_ref instead.
Depends on https://crrev.com/c/1359892

Change-Id: If8ec0bb262035239a1ef77093808196561050c9b
Reviewed-on: https://chromium-review.googlesource.com/c/1359794
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Jao-ke Chin-Lee <jchinlee@chromium.org>