[ObjC ARC] Converts ios/public/provider/chrome/browser/signin:test_support to ARC.

Automatically generated ARCMigrate commit
Notable issues:None
BUG=624363
TEST=None

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ifeddb8439b21e31c2880e07286ab647ac0f6b938
Reviewed-on: https://chromium-review.googlesource.com/844774
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526294}
6 files changed