Add field-trial parameter to enable tests simulating a slow decoder

This CL adds a field trial parameter WebRTC-SlowDownDecoder that is
used to simulate a slow decoder. The parameter specifies how many
extra ms it takes to decode each video frame. This must only be used
in manual testing.

Bug: None
Change-Id: Iad4079100d67b95c224277aaeaf572e38068717f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/151911
Commit-Queue: Johannes Kron <kron@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29153}

modules/video_coding/BUILD.gn

@@ -141,6 +141,7 @@ rtc_static_library("video_coding") {
     "../../api:fec_controller_api",
     "../../api:rtp_headers",
     "../../api/units:data_rate",
+    "../../api/units:time_delta",
     "../../api/video:builtin_video_bitrate_allocator_factory",
     "../../api/video:encoded_frame",
     "../../api/video:video_bitrate_allocator",
@@ -155,6 +156,7 @@ rtc_static_library("video_coding") {
     "../../rtc_base:rtc_numerics",
     "../../rtc_base:rtc_task_queue",
     "../../rtc_base/experiments:alr_experiment",
+    "../../rtc_base/experiments:field_trial_parser",
     "../../rtc_base/experiments:jitter_upper_bound_experiment",
     "../../rtc_base/experiments:rtt_mult_experiment",
     "../../rtc_base/synchronization:sequence_checker",

modules/video_coding/generic_decoder.cc

@@ -18,17 +18,25 @@
 #include "modules/video_coding/include/video_error_codes.h"
 #include "rtc_base/checks.h"
 #include "rtc_base/logging.h"
+#include "rtc_base/thread.h"
 #include "rtc_base/time_utils.h"
 #include "rtc_base/trace_event.h"
 #include "system_wrappers/include/clock.h"
+#include "system_wrappers/include/field_trial.h"
 
 namespace webrtc {
 
 VCMDecodedFrameCallback::VCMDecodedFrameCallback(VCMTiming* timing,
                                                  Clock* clock)
-    : _clock(clock), _timing(timing), _timestampMap(kDecoderFrameMemoryLength) {
+    : _clock(clock),
+      _timing(timing),
+      _timestampMap(kDecoderFrameMemoryLength),
+      _extra_decode_time("t", absl::nullopt) {
   ntp_offset_ =
       _clock->CurrentNtpInMilliseconds() - _clock->TimeInMilliseconds();
+
+  ParseFieldTrial({&_extra_decode_time},
+                  field_trial::FindFullName("WebRTC-SlowDownDecoder"));
 }
 
 VCMDecodedFrameCallback::~VCMDecodedFrameCallback() {}
@@ -64,6 +72,11 @@ int32_t VCMDecodedFrameCallback::Decoded(VideoFrame& decodedImage,
 void VCMDecodedFrameCallback::Decoded(VideoFrame& decodedImage,
                                       absl::optional<int32_t> decode_time_ms,
                                       absl::optional<uint8_t> qp) {
+  // Wait some extra time to simulate a slow decoder.
+  if (_extra_decode_time) {
+    rtc::Thread::SleepMs(_extra_decode_time->ms());
+  }
+
   RTC_DCHECK(_receiveCallback) << "Callback must not be null at this point";
   TRACE_EVENT_INSTANT1("webrtc", "VCMDecodedFrameCallback::Decoded",
                        "timestamp", decodedImage.timestamp());

modules/video_coding/generic_decoder.h

@@ -13,12 +13,14 @@
 
 #include <memory>
 
+#include "api/units/time_delta.h"
 #include "modules/include/module_common_types.h"
 #include "modules/video_coding/encoded_frame.h"
 #include "modules/video_coding/include/video_codec_interface.h"
 #include "modules/video_coding/timestamp_map.h"
 #include "modules/video_coding/timing.h"
 #include "rtc_base/critical_section.h"
+#include "rtc_base/experiments/field_trial_parser.h"
 #include "rtc_base/thread_checker.h"
 
 namespace webrtc {
@@ -71,6 +73,8 @@ class VCMDecodedFrameCallback : public DecodedImageCallback {
   rtc::CriticalSection lock_;
   VCMTimestampMap _timestampMap RTC_GUARDED_BY(lock_);
   int64_t ntp_offset_;
+  // Set by the field trial WebRTC-SlowDownDecoder to simulate a slow decoder.
+  FieldTrialOptional<TimeDelta> _extra_decode_time;
 };
 
 class VCMGenericDecoder {