Introduce WebRTC-VideoEncoderSettings/encoder_thread_limit:X.

As requested by a CEF hosted application (https://crbug.com/1406331)
who want to be able to limit the number of threads in a controlled
environment, this CL adds a flag to control the max limit per encoder.

For plumbing-reasons, this is placed in VideoEncoder::Settings but
with a note that this is considered an experimental API with limited
support. For now only LibvpxVp8Encoder uses it and there are no plans
to roll this out.

I have manually confirmed this is working with printf debugging,
--force-fieldtrials=WebRTC-VideoEncoderSettings/encoder_thread_limit:2
and https://jsfiddle.net/henbos/2bd6m7Lt/

Bug: chromium:1406331
Change-Id: Ib02bd83e2071034874843d3aaa0d3b0adc5bbf46
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/293960
Reviewed-by: Markus Handell <handellm@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#39349}

api/video_codecs/video_encoder.h

@@ -330,6 +330,9 @@ class RTC_EXPORT VideoEncoder {
     Capabilities capabilities;
     int number_of_cores;
     size_t max_payload_size;
+    // Experimental API - currently only supported by LibvpxVp8Encoder.
+    // If set, limits the number of encoder threads.
+    absl::optional<int> encoder_thread_limit;
   };
 
   static VideoCodecVP8 GetDefaultVp8Settings();

modules/video_coding/codecs/vp8/libvpx_vp8_encoder.cc

@@ -609,6 +609,12 @@ int LibvpxVp8Encoder::InitEncode(const VideoCodec* inst,
   // TODO(fbarchard): Consider number of Simulcast layers.
   vpx_configs_[0].g_threads = NumberOfThreads(
       vpx_configs_[0].g_w, vpx_configs_[0].g_h, settings.number_of_cores);
+  if (settings.encoder_thread_limit.has_value()) {
+    RTC_DCHECK_GE(settings.encoder_thread_limit.value(), 1);
+    vpx_configs_[0].g_threads = std::min(
+        vpx_configs_[0].g_threads,
+        static_cast<unsigned int>(settings.encoder_thread_limit.value()));
+  }
 
   // Creating a wrapper to the image - setting image data to NULL.
   // Actual pointer will be set in encode. Setting align to 1, as it

video/video_stream_encoder.cc

@@ -526,6 +526,13 @@ absl::optional<int> ParseVp9LowTierCoreCountThreshold(
   return max_core_count.Get();
 }
 
+absl::optional<int> ParseEncoderThreadLimit(const FieldTrialsView& trials) {
+  FieldTrialOptional<int> encoder_thread_limit("encoder_thread_limit");
+  ParseFieldTrial({&encoder_thread_limit},
+                  trials.Lookup("WebRTC-VideoEncoderSettings"));
+  return encoder_thread_limit.GetOptional();
+}
+
 absl::optional<VideoSourceRestrictions> MergeRestrictions(
     const std::vector<absl::optional<VideoSourceRestrictions>>& list) {
   absl::optional<VideoSourceRestrictions> return_value;
@@ -718,6 +725,7 @@ VideoStreamEncoder::VideoStreamEncoder(
           kSwitchEncoderOnInitializationFailuresFieldTrial)),
       vp9_low_tier_core_threshold_(
           ParseVp9LowTierCoreCountThreshold(field_trials)),
+      experimental_encoder_thread_limit_(ParseEncoderThreadLimit(field_trials)),
       encoder_queue_(std::move(encoder_queue)) {
   TRACE_EVENT0("webrtc", "VideoStreamEncoder::VideoStreamEncoder");
   RTC_DCHECK_RUN_ON(worker_queue_);
@@ -1274,10 +1282,10 @@ void VideoStreamEncoder::ReconfigureEncoder() {
     const size_t max_data_payload_length = max_data_payload_length_ > 0
                                                ? max_data_payload_length_
                                                : kDefaultPayloadSize;
-    if (encoder_->InitEncode(
+    VideoEncoder::Settings settings = VideoEncoder::Settings(
-            &send_codec_,
+        settings_.capabilities, number_of_cores_, max_data_payload_length);
-            VideoEncoder::Settings(settings_.capabilities, number_of_cores_,
+    settings.encoder_thread_limit = experimental_encoder_thread_limit_;
-                                   max_data_payload_length)) != 0) {
+    if (encoder_->InitEncode(&send_codec_, settings) != 0) {
       RTC_LOG(LS_ERROR) << "Failed to initialize the encoder associated with "
                            "codec type: "
                         << CodecTypeToPayloadString(send_codec_.codecType)

video/video_stream_encoder.h

@@ -463,6 +463,7 @@ class VideoStreamEncoder : public VideoStreamEncoderInterface,
   bool switch_encoder_on_init_failures_;
 
   const absl::optional<int> vp9_low_tier_core_threshold_;
+  const absl::optional<int> experimental_encoder_thread_limit_;
 
   // These are copies of restrictions (glorified max_pixel_count) set by
   // a) OnVideoSourceRestrictionsUpdated