Fix collision issues when mixers w/ the same key have errors.

The code was using a map, but we may end up with many mixers with
the same key having errors, which would cause the wrong one to be
leaked. We instead need to keep a list of these.

R=tguilbert

Fixed: 328918906
Change-Id: I4936d88f02dfde3187e0bccf19cd82dafe678161
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5368316
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1271889}