This flag only needs to be set in kOn interlayer prediction mode, because
in all others, if new layer is enabled - a keyframe is generated.
Also, use external reference control in that case, because libvpx creates
rtp-incompatible references in that case.
Bug: webrtc:10180
Change-Id: I0fad188fa8cd424f831bac219769dbad3a788b1d
Reviewed-on: https://webrtc-review.googlesource.com/c/118041
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26316}