Fix stats for encoder target bitrate when target rate is zero.
When the target bitrate is zero, currently  VideoSendStream.Stats.target_media_bitrate_bps show the last set rate before the target was set to zero.

BUG=webrtc::5687 b/29574845

Review-Url: https://codereview.webrtc.org/2122743003
Cr-Original-Commit-Position: refs/heads/master@{#13386}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: f5b2e519b4067e209d2cebbd4b1ef485d2a5c289
18 files changed