- render stream support, required to assess AEC;
- echo path simulation and input mixer, to generate echo and add it to the
speech signal;
- export engine: improved UI, switch to Pandas DataFrames;
- minor design improvements and needed adaptions.
BUG=webrtc:7218
Review-Url: https://codereview.webrtc.org/2813883002
Cr-Commit-Position: refs/heads/master@{#19198}