gerrit_util: Handle already abandoned change

R=keybo@google.com

Bug: 402023825
Change-Id: I104b0868152714384642ff0ea41bb9844e8d54db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6352741
Reviewed-by: Keybo Qian <keybo@google.com>
Auto-Submit: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Keybo Qian <keybo@google.com>

gerrit_util.py

@@ -1433,7 +1433,7 @@ def AbandonChange(host, change, msg=''):
     path = 'changes/%s/abandon' % change
     body = {'message': msg} if msg else {}
     conn = CreateHttpConn(host, path, reqtype='POST', body=body)
-    return ReadHttpJsonResponse(conn)
+    return ReadHttpJsonResponse(conn, accept_statuses=(204, 409))
 
 
 def MoveChange(host, change, destination_branch):