TransportFeedbackPacketLossTrackerTest cosmetic modification

BUG=webrtc:7058

Review-Url: https://codereview.webrtc.org/2751653006
Cr-Original-Commit-Position: refs/heads/master@{#17320}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 6a64bd96bf8cdf795487cd4433415b7f85061927
1 file changed