Prioritize video packets when sending padding or preemptive retransmits.

webrtc/video/video_send_stream.cc

Index: webrtc/video/video_send_stream.cc
diff --git a/webrtc/video/video_send_stream.cc b/webrtc/video/video_send_stream.cc
index 3b4238c47da00fe4c07a3654d2ecdf1a7bedd303..0928e52a7fdd373e30ef3ed8690c917f50d739b5 100644
--- a/webrtc/video/video_send_stream.cc
+++ b/webrtc/video/video_send_stream.cc
@@ -797,9 +797,12 @@ VideoSendStreamImpl::VideoSendStreamImpl(
       config_->periodic_alr_bandwidth_probing);
 
   // RTP/RTCP initialization.
-  for (RtpRtcp* rtp_rtcp : rtp_rtcp_modules_) {
+
+  // We add the highest spatial layer first to ensure it'll be prioritized
+  // when sending padding, with the hope that the packet rate will be smaller,
+  // and that it's more important to protect than the lower layers.
+  for (RtpRtcp* rtp_rtcp : rtp_rtcp_modules_)
     packet_router_->AddRtpModule(rtp_rtcp);
-  }
 
   for (size_t i = 0; i < config_->rtp.extensions.size(); ++i) {
     const std::string& extension = config_->rtp.extensions[i].uri;