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}
diff --git a/webrtc/tools/event_log_visualizer/plot_base.cc b/webrtc/tools/event_log_visualizer/plot_base.cc
index 057a310..63fb092 100644
--- a/webrtc/tools/event_log_visualizer/plot_base.cc
+++ b/webrtc/tools/event_log_visualizer/plot_base.cc
@@ -75,5 +75,11 @@
   series_list_.emplace_back(std::move(time_series));
 }
 
+void Plot::AppendTimeSeriesIfNotEmpty(TimeSeries&& time_series) {
+  if (time_series.points.size() > 0) {
+    series_list_.emplace_back(std::move(time_series));
+  }
+}
+
 }  // namespace plotting
 }  // namespace webrtc