[ObjC ARC] Converts ios/chrome/browser/ui/alert_coordinator:alert_coordinator_internal to ARC.

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

Review-Url: https://codereview.chromium.org/2825023002
Cr-Commit-Position: refs/heads/master@{#465921}
2 files changed