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There are a lot of crashes happening due to an invalid source, so
it appears clear that AudioUnitStop() isn't always working. This
adds a lock so that we do nothing in this case and attempts to
remove the InputProc callback during Close() to prevent UaF on the
stream itself

Fixed: 737527
Change-Id: I97f3442f105317a6214722455dab497280c07336
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532775
Commit-Queue: Olga Sharonova <olka@chromium.org>
Reviewed-by: Olga Sharonova <olka@chromium.org>
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827775}