Overlay REMB in total bitrate graphs in visualization tool.

This doesn't affect the production code.

BUG=webrtc:7726

Review-Url: https://codereview.webrtc.org/2912813002
Cr-Commit-Position: refs/heads/master@{#18400}

webrtc/tools/event_log_visualizer/plot_base.h

@@ -109,6 +109,10 @@ class Plot {
   // Add a new TimeSeries to the plot.
   void AppendTimeSeries(TimeSeries&& time_series);
 
+  // Add a new TimeSeries to the plot if the series contains contains data.
+  // Otherwise, the call has no effect and the timeseries is destroyed.
+  void AppendTimeSeriesIfNotEmpty(TimeSeries&& time_series);
+
  protected:
   float xaxis_min_;
   float xaxis_max_;