in the case of separate encoders. drive-by: add helper function to expect per-layer keyframes BUG=chromium:1354101 Change-Id: Ib645a621add899f035bea319f035dcb0b2617510 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/281002 Reviewed-by: Erik Språng <sprang@webrtc.org> Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org> Commit-Queue: Philipp Hancke <phancke@microsoft.com> Cr-Commit-Position: refs/heads/main@{#38734}