Format trace file names so they are easier to organize

packages/flutter/lib/src/widgets/statistics_overlay.dart

@@ -64,7 +64,7 @@ class StatisticsOverlay extends LeafRenderObjectWidget {
   /// 0 disables all capture.
   ///
   /// Captured traces are placed on your device in the application documents
-  /// directory in this form "trace_<timestamp_since_app_start>.skp". These can
+  /// directory in this form "trace_<collection_time>.skp". These can
   /// be viewed in the Skia debugger.
   ///
   /// Notes: