Avoid repeated semi-expensive field_trials read in frame_buffer2.cc

Bug: webrtc:10202
Change-Id: Ib8bfe7c1d62bc5091a8bfb2ce137ba749f9042e6
Reviewed-on: https://webrtc-review.googlesource.com/c/117361
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Commit-Queue: Elad Alon <eladalon@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26261}

modules/video_coding/frame_buffer2.h

@@ -173,6 +173,8 @@ class FrameBuffer {
   VCMReceiveStatisticsCallback* const stats_callback_;
   int64_t last_log_non_decoded_ms_ RTC_GUARDED_BY(crit_);
 
+  const bool add_rtt_to_playout_delay_;
+
   RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(FrameBuffer);
 };