media::AudioProcessor::OnPlayoutData: receive data as const ref instead of pointer

The function will be called via two APIs, one that takes a const ref:
https://source.chromium.org/chromium/chromium/src/+/main:services/audio/reference_output.h;l=22;drc=2c0bab7e24d5eceb148c2a43c0c1edd8aafb290f
... and the current client, which has a pointer to non-const:
https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/platform/webrtc/webrtc_source.h;l=65;drc=2c0bab7e24d5eceb148c2a43c0c1edd8aafb290f

Changing media::AudioProcessor::OnPlayoutData to const ref makes it compatible with both and reduces ambiguity (i.e., "is the bus nullptr?")

Bug: chromium:1215061
Change-Id: I8e79361cb4a203a00df6fd5b70c6140be62927ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3448370


Reviewed-by: Olga Sharonova <olka@chromium.org>
Commit-Queue: Sam Zackrisson <saza@chromium.org>
Cr-Commit-Position: refs/heads/main@{#968353}