Fill drops with last decoded frame to make them look like freeze at playback and to keep decoded spatial layers in sync. Bug: none Change-Id: I65f7c21100985c22932a1edd441b6c724833c11e Reviewed-on: https://webrtc-review.googlesource.com/73685 Commit-Queue: Sergey Silkin <ssilkin@webrtc.org> Reviewed-by: Rasmus Brandt <brandtr@webrtc.org> Cr-Commit-Position: refs/heads/master@{#23076}