The MediaStreamAudioProcessor has an internal buffer, render_fifo_, to ensure the APM gets 10 ms chunks of audio. It appears redundant, because it is always fed 10 ms chunks provided by WebRTC. It should be removed in order to:
2) make it easier to switch to using the APM via ProcessReverseStream over the non-modifying AnalyzeReverseStream;
1) reduce code complexity;
3) avoid copying the audio buffer an extra time.

Location of buffer:
https://cs.chromium.org/chromium/src/content/renderer/media/media_stream_audio_processor.cc?q=media_stream_audio_processor&sq=package:chromium&dr=C&l=526

The only non-unittest caller of MediaStreamAudioProcessor::OnPlayoutData is WebRtcAudioDeviceImpl::RenderData, which explicitly DCHECKs that the audio is 10 ms long.
https://cs.chromium.org/chromium/src/content/renderer/media/webrtc_audio_device_impl.cc?sq=package:chromium&dr=C&l=74

All occurrences in the unit tests (callers of ProcessDataAndVerifyFormat) use 10 ms frames as well.
https://cs.chromium.org/chromium/src/content/renderer/media/media_stream_audio_processor_unittest.cc?sq=package:chromium&dr=C&l=93
(expected_output_buffer_size is always given as sample_rate / 100, i.e. 10 ms)
 

The MediaStreamAudioProcessor has an internal buffer, render_fifo_ [1], to ensure the APM gets 10 ms chunks of audio. It appears redundant, because it is always fed 10 ms chunks provided by WebRTC. It should be removed in order to:
2) make it easier to switch to using the APM via ProcessReverseStream over the non-modifying AnalyzeReverseStream;
1) reduce code complexity;
3) avoid copying the audio buffer an extra time.

The only non-unittest caller of MediaStreamAudioProcessor::OnPlayoutData is WebRtcAudioDeviceImpl::RenderData [2], which explicitly DCHECKs that the audio is 10 ms long.

The unit tests (via ProcessDataAndVerifyFormat [3]) use the same frame size for capture as for render, not necessarily 10 ms.

[1] https://cs.chromium.org/chromium/src/content/renderer/media/media_stream_audio_processor.cc?q=media_stream_audio_processor&sq=package:chromium&dr=C&l=526

[2] https://cs.chromium.org/chromium/src/content/renderer/media/webrtc_audio_device_impl.cc?sq=package:chromium&dr=C&l=74

[3] https://cs.chromium.org/chromium/src/content/renderer/media/media_stream_audio_processor_unittest.cc?sq=package:chromium&dr=C&l=98