Improve usability of groupby-picker.

tracing/tracing/ui/side_panel/file_size_stats_side_panel.html

Index: tracing/tracing/ui/side_panel/file_size_stats_side_panel.html
diff --git a/tracing/tracing/ui/side_panel/file_size_stats_side_panel.html b/tracing/tracing/ui/side_panel/file_size_stats_side_panel.html
index 21c7b307ee460679ff4df1682a17a217dfa8acec..6e87fbc8b9f2a9250487b74989c88a9380ab5a8e 100644
--- a/tracing/tracing/ui/side_panel/file_size_stats_side_panel.html
+++ b/tracing/tracing/ui/side_panel/file_size_stats_side_panel.html
@@ -77,7 +77,11 @@ found in the LICENSE file.
           dataFn: function(eventStat) { return eventStat.title; }
         }
       ];
-      this.$.picker.defaultGroupKeys = ['phase', 'title'];
+      // If the picker did not restore currentGroupKeys from Settings,
+      // then set default currentGroupKeys.
+      if (this.$.picker.currentGroupKeys.length === 0) {
+        this.$.picker.currentGroupKeys = ['phase', 'title'];
+      }
       this.$.picker.addEventListener('current-groups-changed',
                                      this.updateContents_.bind(this));
     },