Ignore inter-frame delay stats samples when stream is inactive

BUG=webrtc:7694

Review-Url: https://codereview.webrtc.org/3002103002
Cr-Original-Commit-Position: refs/heads/master@{#19453}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 3e86e7eec727b9c2348a92ca3282d9066b4f9e9a
4 files changed