Stash non layer-sync frames if we have not yet received an earlier frame for this layer.

webrtc/modules/video_coding/rtp_frame_reference_finder.cc

 /*
  *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 273 matching lines @@
     frame->num_references = 1;
     frame->references[0] = layer_info_it->second[0];
 
     CompletedFrameVp8(std::move(frame));
     return;
   }
 
   // Find all references for this frame.
   frame->num_references = 0;
   for (uint8_t layer = 0; layer <= codec_header.temporalIdx; ++layer) {
-    RTC_DCHECK_NE(-1, layer_info_it->second[layer]);
+    // If we have not yet received a previous frame on this temporal layer,
+    // stash this frame.
+    if (layer_info_it->second[layer] == -1) {
+      stashed_frames_.push_back(std::move(frame));
+      return;
+    }
 
     // If we have not yet received a frame between this frame and the referenced
     // frame then we have to wait for that frame to be completed first.
     auto not_received_frame_it =
         not_yet_received_frames_.upper_bound(layer_info_it->second[layer]);
     if (not_received_frame_it != not_yet_received_frames_.end() &&
         AheadOf<uint16_t, kPicIdLength>(frame->picture_id,
                                         *not_received_frame_it)) {
       stashed_frames_.push_back(std::move(frame));
       return;
@@ skipping 259 matching lines @@
   if (AheadOf<uint16_t, kPicIdLength>(picture_id, unwrap_truncated))
     last_unwrap_ = Add<1 << 16>(last_unwrap_, diff);
   else
     last_unwrap_ = Subtract<1 << 16>(last_unwrap_, diff);
 
   return last_unwrap_;
 }
 
 }  // namespace video_coding
 }  // namespace webrtc