[MacOS Host] Fixing a memory leak in the alert dialog

Follow-up CL fixing a leak identified post check-in.

Bug: 902041
Change-Id: I733cc6037f5c29f0decabae72d51a6470de17f91
Reviewed-on: https://chromium-review.googlesource.com/c/1355332
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612298}
1 file changed