Report time_between_freezes as test duration when there were no freezes in the call. Bug: webrtc:10138 Change-Id: I3d99be4b714f1b1d13e7b8b7055b368a20859490 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/141665 Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org> Commit-Queue: Artem Titov <titovartem@webrtc.org> Cr-Commit-Position: refs/heads/master@{#28248}