This allows mixing different number of streams depending on the client's capabilities. This CL adds `WebRTC.Audio.AudioMixer.NumIncomingActiveStreams2`, which is defined in [1], since the histogram is not logged anymore as enum. [1] https://chromium-review.googlesource.com/c/chromium/src/+/2883627 Bug: webrtc:12746 Change-Id: I0d9b3888f0f95269806539e33b56619b757a5c68 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/218160 Reviewed-by: Alessio Bazzica <alessiob@webrtc.org> Reviewed-by: Minyue Li <minyue@webrtc.org> Reviewed-by: Johannes Kron <kron@webrtc.org> Commit-Queue: Doudou Kisabaka <doudouk@google.com> Cr-Commit-Position: refs/heads/master@{#34024}