Fix the video buffer size should take rtt into consideration

BUG=webrtc:8010

Review-Url: https://codereview.webrtc.org/2980413002
Cr-Commit-Position: refs/heads/master@{#19285}

webrtc/modules/video_coding/frame_buffer2.h

@@ -74,6 +74,9 @@ class FrameBuffer {
   // return immediately.
   void Stop();
 
+  // Updates the RTT for jitter buffer estimation.
+  void UpdateRtt(int64_t rtt_ms);
+
  private:
   struct FrameKey {
     FrameKey() : picture_id(0), spatial_layer(0) {}