Build superframe out of the nearest non-dropped base layer and current layer. Bug: none Change-Id: I26720ea6de44f27046208b220d03942cd2a3d6c7 Reviewed-on: https://webrtc-review.googlesource.com/64921 Reviewed-by: Rasmus Brandt <brandtr@webrtc.org> Commit-Queue: Sergey Silkin <ssilkin@webrtc.org> Cr-Commit-Position: refs/heads/master@{#22631}