Event log visualizer TimeSeries is now created on the stack and then moved into the vector of series.

BUG=none

Review-Url: https://codereview.webrtc.org/2824973003
Cr-Commit-Position: refs/heads/master@{#17763}
diff --git a/webrtc/tools/event_log_visualizer/plot_base.cc b/webrtc/tools/event_log_visualizer/plot_base.cc
index baee3df..057a310 100644
--- a/webrtc/tools/event_log_visualizer/plot_base.cc
+++ b/webrtc/tools/event_log_visualizer/plot_base.cc
@@ -71,14 +71,8 @@
   title_ = title;
 }
 
-TimeSeries* Plot::AddTimeSeries(const char* label, PlotStyle style) {
-  series_list_.emplace_back(label, style);
-  return &series_list_.back();
-}
-
-TimeSeries* Plot::AddTimeSeries(const std::string& label, PlotStyle style) {
-  series_list_.emplace_back(label, style);
-  return &series_list_.back();
+void Plot::AppendTimeSeries(TimeSeries&& time_series) {
+  series_list_.emplace_back(std::move(time_series));
 }
 
 }  // namespace plotting