Always write scalability structure no matter whether flexible mode is enabled or not. Bug: webrtc:9658 Change-Id: I6c3d5c8e46046dfe89e8ec38536d71b195e80593 Reviewed-on: https://webrtc-review.googlesource.com/95723 Reviewed-by: Åsa Persson <asapersson@webrtc.org> Commit-Queue: Sergey Silkin <ssilkin@webrtc.org> Cr-Commit-Position: refs/heads/master@{#24406}