Control rtt_mult addition cap via experiment.

Bug: webrtc:10717
Change-Id: I68f7d8216e1a1611e692dd82ba96890cad98c7de
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/140284
Commit-Queue: Michael Horowitz <mhoro@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28191}

modules/video_coding/frame_buffer2.h

@@ -195,6 +195,9 @@ class FrameBuffer {
 
   const bool add_rtt_to_playout_delay_;
 
+  // rtt_mult experiment settings.
+  const absl::optional<RttMultExperiment::Settings> rtt_mult_settings_;
+
   RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(FrameBuffer);
 };