zhihuang e9e94c3fee Return false if PeerConnection::GetStats() is called on invalid tracks
Before calling StatsCollctor::GetStats() in PeerConnection::GetStats(), check if the track is valid. If not, return false.
A track is invalid if it is not a nullptr and there is no report data for it.

BUG=webrtc:6652

Review-Url: https://codereview.webrtc.org/2470023004
Cr-Commit-Position: refs/heads/master@{#14934}
2016-11-04 18:38:19 +00:00
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