Fix documentation of MediaStreamManager::audio_service_listener()

The DCHECK ensures it's called on the UI thread, yet the comment
says it must be called on the IO thread. As this is the only
public method in MediaStreamManager that is UI-thread bound,
it's an open question as to whether the code
is advisable (possibly yes). But at any rate,
accurate documentation is better than the alternative.

Bug: None
Change-Id: If3f851c4552e1a7f6be43ce8d13071474e8620bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3687880
Reviewed-by: Simon Hangl <simonha@google.com>
Commit-Queue: Simon Hangl <simonha@google.com>
Auto-Submit: Elad Alon <eladalon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1011359}