Erase pending frames after they had been used. Bug: chromium:956510 Change-Id: I55eec75dab9bbe350dd1b089953bdc2979d989fd Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/134641 Commit-Queue: Elad Alon <eladalon@webrtc.org> Reviewed-by: Rasmus Brandt <brandtr@webrtc.org> Cr-Commit-Position: refs/heads/master@{#27811}