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_unittest.cc

@@ -56,7 +56,8 @@ class VCMTimingFake : public VCMTiming {
   }
 
   int64_t MaxWaitingTime(int64_t render_time_ms,
-                         int64_t now_ms) const override {
+                         int64_t now_ms,
+                         bool too_many_frames_queued) const override {
     return render_time_ms - now_ms - kDecodeTime;
   }