Ignore zero bitrate updates in the UMA BWE stats as they represent network being down and would bias the stats.
Ignore zero bitrate updates in the UMA BWE stats as they represent network being down and would bias the stats.

BUG=

Review-Url: https://codereview.webrtc.org/2161053002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#13518}

TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc