Store the last known throughput estimate and use that if we're lacking a new measurement. Bug: webrtc:9363 Change-Id: Ib7a9a495b446bd0f5799382cc095ccff894e0c2b Reviewed-on: https://webrtc-review.googlesource.com/81749 Commit-Queue: Anastasia Koloskova <koloskova@webrtc.org> Reviewed-by: Sebastian Jansson <srte@webrtc.org> Reviewed-by: Björn Terelius <terelius@webrtc.org> Cr-Commit-Position: refs/heads/master@{#23565}