Add new step graph type to event log visualization tool

webrtc/tools/event_log_visualizer/chart.proto

 // Describes a chart generated from WebRTC event log data.
 syntax = "proto3";
 option optimize_for = LITE_RUNTIME;
 
 package webrtc.analytics;
 
 message ChartStyle {
   enum Type {
     UNDEFINED = 0;
     LINE_CHART = 1;
     BAR_CHART = 2;
+    LINE_STEP_CHART = 3;
   }
 }
 
 message DataSet {
   repeated float x_values = 1;
   repeated float y_values = 2;
   string label = 3;
   ChartStyle.Type style = 4;
   bool highlight_points = 5;
 }
 
 message Chart {
   repeated DataSet data_sets = 1;
   float xaxis_min = 2;
   float xaxis_max = 3;
   string xaxis_label = 4;
   float yaxis_min = 5;
   float yaxis_max = 6;
   string yaxis_label = 7;
   string title = 8;
 }
 
 message ChartCollection {
   repeated Chart charts = 1;
-}
+}