GitLab

- Add MixerUnderrun proto for reporting mixer input/output underruns
  to OutputStreamConnections
- Add MixerInput::Source::OnOutputUnderrun virtual method, called on
  every mixer input whenever StreamMixer detects an output underrun
- Add OutputStreamConnection::Delegate::OnMixerUnderrun virtual method,
  called in response to MixerUnderrun proto

      simulated poor network conditions

Bug: b/142084721
Test: manual verification of values logged from RecordUnderruns() in
Merge-With: eureka-internal/350603
Change-Id: I6ad4dcdd573a5a6b9d89baf5f15abac0a109ee92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1992068Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Marcin Radomski <dextero@google.com>
Cr-Commit-Position: refs/heads/master@{#732432}