Revert "Replaced temporal_layer_thresholds_bps[] field with num_temporal_layers."

This reverts commit d2ed0a4c9e7f04060d8e3358eb0006c31579bb86.

Reason for revert: Breaks downstream projects.

Original change's description:
> Replaced temporal_layer_thresholds_bps[] field with num_temporal_layers.
> 
> temporal_layer_thresholds_bps served only one purpose: its size was used
> to infer number of temporal layers. I replaced it with num_temporal_layers,
> which does what is says.
> 
> The practical reason for this change is the need to have possibility to
> distinguish between cases when VP9 SVC temporal layering was/not set
> through field trial. That was not possible with
> temporal_layer_thresholds_bps[] because empty vector means 1 temporal
> layer.
> 
> Bug: webrtc:8518
> Change-Id: I275ec3a8c74e8ba409eb049878199f132a20ec51
> Reviewed-on: https://webrtc-review.googlesource.com/58084
> Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
> Reviewed-by: Erik Språng <sprang@webrtc.org>
> Reviewed-by: Stefan Holmer <stefan@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#22230}

TBR=sprang@webrtc.org,stefan@webrtc.org,ssilkin@webrtc.org

Change-Id: Ic2940f7f78a74312170940d51ad8967cde8ad42f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:8518
Reviewed-on: https://webrtc-review.googlesource.com/58902
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22234}
diff --git a/modules/video_coding/video_codec_initializer.cc b/modules/video_coding/video_codec_initializer.cc
index 3758823..d81358d 100644
--- a/modules/video_coding/video_codec_initializer.cc
+++ b/modules/video_coding/video_codec_initializer.cc
@@ -23,6 +23,15 @@
 #include "system_wrappers/include/clock.h"
 
 namespace webrtc {
+namespace {
+bool TemporalLayersConfigured(const std::vector<VideoStream>& streams) {
+  for (const VideoStream& stream : streams) {
+    if (stream.temporal_layer_thresholds_bps.size() > 0)
+      return true;
+  }
+  return false;
+}
+}  // namespace
 
 bool VideoCodecInitializer::SetupCodec(
     const VideoEncoderConfig& config,
@@ -120,8 +129,10 @@
       break;
     case VideoEncoderConfig::ContentType::kScreen:
       video_codec.mode = kScreensharing;
-      if (!streams.empty() && streams[0].num_temporal_layers == 2) {
-        video_codec.targetBitrate = streams[0].target_bitrate_bps / 1000;
+      if (!streams.empty() &&
+          streams[0].temporal_layer_thresholds_bps.size() == 1) {
+        video_codec.targetBitrate =
+            streams[0].temporal_layer_thresholds_bps[0] / 1000;
       }
       break;
   }
@@ -131,16 +142,12 @@
 
   switch (video_codec.codecType) {
     case kVideoCodecVP8: {
-      if (!config.encoder_specific_settings) {
+      if (!config.encoder_specific_settings)
         *video_codec.VP8() = VideoEncoder::GetDefaultVp8Settings();
-      }
-
       video_codec.VP8()->numberOfTemporalLayers = static_cast<unsigned char>(
-          streams.back().num_temporal_layers.value_or(
-              video_codec.VP8()->numberOfTemporalLayers));
-      RTC_DCHECK_GE(video_codec.VP8()->numberOfTemporalLayers, 1);
+          streams.back().temporal_layer_thresholds_bps.size() + 1);
 
-      if (nack_enabled && video_codec.VP8()->numberOfTemporalLayers == 1) {
+      if (nack_enabled && !TemporalLayersConfigured(streams)) {
         RTC_LOG(LS_INFO)
             << "No temporal layers and nack enabled -> resilience off";
         video_codec.VP8()->resilience = kResilienceOff;
@@ -148,10 +155,8 @@
       break;
     }
     case kVideoCodecVP9: {
-      if (!config.encoder_specific_settings) {
+      if (!config.encoder_specific_settings)
         *video_codec.VP9() = VideoEncoder::GetDefaultVp9Settings();
-      }
-
       if (video_codec.mode == kScreensharing &&
           config.encoder_specific_settings) {
         video_codec.VP9()->flexibleMode = true;
@@ -159,13 +164,10 @@
         RTC_DCHECK_EQ(1, video_codec.VP9()->numberOfTemporalLayers);
         RTC_DCHECK_EQ(2, video_codec.VP9()->numberOfSpatialLayers);
       }
-
       video_codec.VP9()->numberOfTemporalLayers = static_cast<unsigned char>(
-          streams.back().num_temporal_layers.value_or(
-              video_codec.VP9()->numberOfTemporalLayers));
-      RTC_DCHECK_GE(video_codec.VP9()->numberOfTemporalLayers, 1);
+          streams.back().temporal_layer_thresholds_bps.size() + 1);
 
-      if (nack_enabled && video_codec.VP9()->numberOfTemporalLayers == 1 &&
+      if (nack_enabled && !TemporalLayersConfigured(streams) &&
           video_codec.VP9()->numberOfSpatialLayers == 1) {
         RTC_LOG(LS_INFO) << "No temporal or spatial layers and nack enabled -> "
                          << "resilience off";
@@ -237,8 +239,8 @@
     sim_stream->targetBitrate = streams[i].target_bitrate_bps / 1000;
     sim_stream->maxBitrate = streams[i].max_bitrate_bps / 1000;
     sim_stream->qpMax = streams[i].max_qp;
-    sim_stream->numberOfTemporalLayers =
-        static_cast<unsigned char>(streams[i].num_temporal_layers.value_or(1));
+    sim_stream->numberOfTemporalLayers = static_cast<unsigned char>(
+        streams[i].temporal_layer_thresholds_bps.size() + 1);
     sim_stream->active = streams[i].active;
 
     video_codec.width =