Reland "[VP9] Shift spatial layers on RTP level to always start from 0."

This reverts commit 2181228624d1be60903c4e3352629290b9c3b27a.

Reason for revert: Reland without changes as it's not the root cause.

Original change's description:
> Revert "[VP9] Shift spatial layers on RTP level to always start from 0."
> 
> This reverts commit 2e73a3d1e9298da6a010cd638f08f36abeba11e2.
> 
> Reason for revert: Fuzzer found some issues.
> 
> Original change's description:
> > [VP9] Shift spatial layers on RTP level to always start from 0.
> > 
> > This CL uses |width| and |height| in RTPVideoHeaderVP9 to pass information
> > about enabled layers from encoder to packetizer.
> > 
> > Bug: webrtc:11319
> > Change-Id: Idc1c337f8dfb3f7631506acb784d2a634b41b955
> > Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/167724
> > Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
> > Reviewed-by: Niels Moller <nisse@webrtc.org>
> > Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
> > Cr-Commit-Position: refs/heads/master@{#30428}
> 
> TBR=danilchap@webrtc.org,ilnik@webrtc.org,nisse@webrtc.org
> 
> # Not skipping CQ checks because original CL landed > 1 day ago.
> 
> Bug: webrtc:11319
> Change-Id: I27a7e82737fa604b8ab769ce6503fa93e46f4e86
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/168123
> Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
> Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#30448}

TBR=danilchap@webrtc.org,ilnik@webrtc.org,nisse@webrtc.org

Change-Id: Ibcd9b6a075ee08c9402de8b0b9d99d77bf59d0ef
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:11319
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/168185
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30450}

modules/video_coding/include/video_codec_interface.h

@@ -69,6 +69,7 @@ struct CodecSpecificInfoVP9 {
 
   // SS data.
   size_t num_spatial_layers;  // Always populated.
+  size_t first_active_layer;
   bool spatial_layer_resolution_present;
   uint16_t width[kMaxVp9NumberOfSpatialLayers];
   uint16_t height[kMaxVp9NumberOfSpatialLayers];