Use rtc::scoped_refptr to reference webrtc::AudioProcessing

Bug: webrtc:12701
Change-Id: I5e11a9856842ab871fdf7ce1cf0e398e8e5da134
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3031070


Reviewed-by: Olga Sharonova <olka@chromium.org>
Commit-Queue: Niels Möller <nisse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#910806}