Enforce VideoEncoderConfig.num_temporal_layers >= 1.

This change clarifies the semantics of this field:
  unset: Depends on context.
  == 0: Invalid.
  == 1: No temporal layering.
  >= 2: Temporal layering.

We should try to remove the wrapping optional later.

Bug: webrtc:11297
Change-Id: Id765f2dc1d31a4ba3cd424978ac6054cd60152ea
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/166528
Commit-Queue: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30336}
diff --git a/api/video_codecs/video_encoder_config.h b/api/video_codecs/video_encoder_config.h
index 399b3cc..ef8db10 100644
--- a/api/video_codecs/video_encoder_config.h
+++ b/api/video_codecs/video_encoder_config.h
@@ -24,28 +24,47 @@
 
 namespace webrtc {
 
+// The |VideoStream| struct describes a simulcast layer, or "stream".
 struct VideoStream {
   VideoStream();
   ~VideoStream();
   VideoStream(const VideoStream& other);
   std::string ToString() const;
 
+  // Width in pixels.
   size_t width;
+
+  // Height in pixels.
   size_t height;
+
+  // Frame rate in fps.
   int max_framerate;
 
+  // Bitrate, in bps, for the stream.
   int min_bitrate_bps;
   int target_bitrate_bps;
   int max_bitrate_bps;
+
   // Scaling factor applied to the stream size.
   // |width| and |height| values are already scaled down.
   double scale_resolution_down_by;
+
+  // Maximum Quantization Parameter to use when encoding the stream.
   int max_qp;
 
+  // Determines the number of temporal layers that the stream should be
+  // encoded with. This value should be greater than zero.
+  // TODO(brandtr): This class is used both for configuring the encoder
+  // (meaning that this field _must_ be set), and for signaling the app-level
+  // encoder settings (meaning that the field _may_ be set). We should separate
+  // this and remove this optional instead.
   absl::optional<size_t> num_temporal_layers;
 
+  // The priority of this stream, to be used when allocating resources
+  // between multiple streams.
   absl::optional<double> bitrate_priority;
 
+  // If this stream is enabled by the user, or not.
   bool active;
 };