Convenience functions to set axis properties in visualization tool.

Also makes the properties protected, as previously requested by Stefan.

Review-Url: https://codereview.webrtc.org/2179223003
Cr-Commit-Position: refs/heads/master@{#13593}

webrtc/tools/event_log_visualizer/analyzer.h

@@ -115,6 +115,9 @@ class EventLogAnalyzer {
   // First and last events of the log.
   uint64_t begin_time_;
   uint64_t end_time_;
+
+  // Duration (in seconds) of log file.
+  float call_duration_s_;
 };
 
 }  // namespace plotting