Add fieldtrial to enable minimum pacing of video frames

If the RTP header extension playout-delay is used and set
to min=0, max>=0, frames are scheduled to be decoded as
soon as possible. There's a risk that too many frames are
sent to the decoder at once, which may cause problems
further down in the video pipeline.

This CL adds the fieldtrial WebRTC-ZeroPlayoutDelay with
the parameter min_pacing that determines the minimum
pacing interval between two frames scheduled for
decoding.

Bug: None
Change-Id: I471f7718761cfce9789b3aa8adea3e8a16ecb2fd
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/223742
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Johannes Kron <kron@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34387}

modules/video_coding/frame_buffer2_unittest.cc

@@ -55,8 +55,7 @@ class VCMTimingFake : public VCMTiming {
     return last_ms_;
   }
 
-  int64_t MaxWaitingTime(int64_t render_time_ms,
-                         int64_t now_ms) const override {
+  int64_t MaxWaitingTime(int64_t render_time_ms, int64_t now_ms) override {
     return render_time_ms - now_ms - kDecodeTime;
   }