Mitigate RTP pic ID errors in VP9 SVC non-flexible mode
Mitigate RTP pic ID errors in VP9 SVC non-flexible mode

The code changes in this CL configure VP9 SVC to drop a superframe when
the spatial base layer is dropped and to not drop upper spatial layers
when the spatial base layer is not dropped. The changes are effective in
non-flexible mode when codec_.mode == kRealtimeVideo and
number of spatial layers > 1.

Bug: none
Change-Id: I27481b78f733cfc6c007d1ad9f45d69263853149
Reviewed-on: https://webrtc-review.googlesource.com/74261
Commit-Queue: Michael Horowitz <1102f2de1330857c46b7dcc057d19df8c909838c@webrtc.org>
Reviewed-by: Sergey Silkin <fc631354f276613a5410ea259cf4618b2846d39b@webrtc.org>
Reviewed-by: Rasmus Brandt <9545de3a34b925a736ae2b5ee593f3033f743a28@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#23127}
TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc