Add max pre-decode queue size threshold for pacing

When pacing is enabled for the low latency rendering path,
frames are sent to the decoder in regular intervals. In case of a
jitter, these frames intervals could add up to create a large latency.
Hence, disable frame pacing if the pre-decode queue grows beyond the
threshold. The threshold for when to disable frame pacing is set
through a field trial. The default value is high enough so that
the behavior is not changed unless the field trial is specified.

Bug: chromium:1237402
Change-Id: I901fd579f68da286eca3d654118f60d3c55e21ce
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/228241
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Johannes Kron <kron@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34705}

modules/video_coding/frame_buffer2.h

@@ -25,6 +25,7 @@
 #include "modules/video_coding/jitter_estimator.h"
 #include "modules/video_coding/utility/decoded_frames_history.h"
 #include "rtc_base/event.h"
+#include "rtc_base/experiments/field_trial_parser.h"
 #include "rtc_base/experiments/rtt_mult_experiment.h"
 #include "rtc_base/numerics/sequence_number_util.h"
 #include "rtc_base/synchronization/mutex.h"
@@ -188,6 +189,13 @@ class FrameBuffer {
 
   // rtt_mult experiment settings.
   const absl::optional<RttMultExperiment::Settings> rtt_mult_settings_;
+
+  // Maximum number of frames in the decode queue to allow pacing. If the
+  // queue grows beyond the max limit, pacing will be disabled and frames will
+  // be pushed to the decoder as soon as possible. This only has an effect
+  // when the low-latency rendering path is active, which is indicated by
+  // the frame's render time == 0.
+  FieldTrialParameter<unsigned> zero_playout_delay_max_decode_queue_size_;
 };
 
 }  // namespace video_coding