Add audio_level member to RtpSource and set it from RtpReceiverImpl::IncomingRtpPacket.

BUG=webrtc:7987

Review-Url: https://codereview.webrtc.org/3000713002
Cr-Original-Commit-Position: refs/heads/master@{#19503}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 2b706343de6f79839f46d12008ec3c8062bb94bc
4 files changed