SignalProcessingUtils.MixSignals() now allows different padding options. This CL also adds more unit tests for SignalProcessingUtils.MixSignals(). Bug: webrtc:7494 Change-Id: Id62fe9998e512c275cb6399e0aedf11f23a9f36e Reviewed-on: https://webrtc-review.googlesource.com/5780 Commit-Queue: Alessio Bazzica <alessiob@webrtc.org> Reviewed-by: Alex Loiko <aleloi@webrtc.org> Cr-Commit-Position: refs/heads/master@{#20122}