Fix race in VideoSendStreamTest.RtcpSenderReportContainsMediaBytesSent.
Fix race in VideoSendStreamTest.RtcpSenderReportContainsMediaBytesSent.

BUG=webrtc:5194

Review URL: https://codereview.webrtc.org/1434963002

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#10602}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 4b56904b70b2ad38c0790c0159819e89c05513b7

jchavanton/webrtc,jchavanton/webrtc,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,aleonliao/webrtc-trunk,aleonliao/webrtc-trunk,aleonliao/webrtc-trunk,Alkalyne/webrtctrunk,jchavanton/webrtc,aleonliao/webrtc-trunk,jchavanton/webrtc,Alkalyne/webrtctrunk,jchavanton/webrtc,Alkalyne/webrtctrunk,jchavanton/webrtc,aleonliao/webrtc-trunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,jchavanton/webrtc,Alkalyne/webrtctrunk,aleonliao/webrtc-trunk