Set priority of rtp_send_controller task queue to high for low latency mode. (#177)

Author: jianjunz

call/rtp_transport_controller_send.cc

@@ -36,6 +36,7 @@ static const int64_t kRetransmitWindowSizeMs = 500;
 static const size_t kMaxOverheadBytes = 500;
 
 constexpr TimeDelta kPacerQueueUpdateInterval = TimeDelta::Millis(25);
+const char kLowLatencyStreaming[] = "OWT-LowLatencyMode";
 
 TargetRateConstraints ConvertConstraints(int min_bitrate_bps,
                                          int max_bitrate_bps,
@@ -54,6 +55,11 @@ TargetRateConstraints ConvertConstraints(int min_bitrate_bps,
   return msg;
 }
 
+bool LowLatencyStreamingEnabled() {
+  std::string trial = webrtc::field_trial::FindFullName(kLowLatencyStreaming);
+  return trial.find("Enabled") == 0;
+}
+
 TargetRateConstraints ConvertConstraints(const BitrateConstraints& contraints,
                                          Clock* clock) {
   return ConvertConstraints(contraints.min_bitrate_bps,
@@ -119,10 +125,13 @@ RtpTransportControllerSend::RtpTransportControllerSend(
       congestion_window_size_(DataSize::PlusInfinity()),
       is_congested_(false),
       retransmission_rate_limiter_(clock, kRetransmitWindowSizeMs),
-	  // TODO: Potentially change priority when low latency mode is enabled.
-      task_queue_(task_queue_factory->CreateTaskQueue(
-          "rtp_send_controller",
-          TaskQueueFactory::Priority::NORMAL)),
+      task_queue_(LowLatencyStreamingEnabled()
+                      ? task_queue_factory->CreateTaskQueue(
+                            "rtp_send_controller",
+                            TaskQueueFactory::Priority::HIGH)
+                      : task_queue_factory->CreateTaskQueue(
+                            "rtp_send_controller",
+                            TaskQueueFactory::Priority::NORMAL)),
       field_trials_(trials) {
   ParseFieldTrial({&relay_bandwidth_cap_},
                   trials.Lookup("WebRTC-Bwe-NetworkRouteConstraints"));