These tests were flaky since a paced packet could arrive between consecutive calls to NumRtpPackets or NumRtpBytes. BUG=webrtc:5193 R=stefan@webrtc.org TBR=pthatcher@webrtc.org Review URL: https://codereview.webrtc.org/1436753003 . Cr-Commit-Position: refs/heads/master@{#10603}