This adds an interface for accessing stats on the capture stream, and adds a level estimator to report one of the stats. Bug: webrtc:9947 Change-Id: Id472534fa2e04d46c9ab700671f620584a246afb Reviewed-on: https://webrtc-review.googlesource.com/c/109587 Commit-Queue: Sam Zackrisson <saza@webrtc.org> Reviewed-by: Per Åhgren <peah@webrtc.org> Cr-Commit-Position: refs/heads/master@{#25786}