Added standard deviation to rasterizer results. (#131781)

This calculation is important if you want to calculate the probability that one thing is better than another.

dev/devicelab/lib/tasks/gallery.dart

@@ -189,6 +189,7 @@ class GalleryTransitionTest {
         '90th_percentile_frame_build_time_millis',
         '99th_percentile_frame_build_time_millis',
         'average_frame_rasterizer_time_millis',
+        'stddev_frame_rasterizer_time_millis',
         'worst_frame_rasterizer_time_millis',
         '90th_percentile_frame_rasterizer_time_millis',
         '99th_percentile_frame_rasterizer_time_millis',

packages/flutter_driver/lib/src/driver/timeline_summary.dart

@@ -82,6 +82,20 @@ class TimelineSummary {
     return _averageInMillis(_extractGpuRasterizerDrawDurations());
   }
 
+  /// Standard deviation amount of time spent per frame in the engine rasterizer.
+  ///
+  /// Throws a [StateError] if this summary contains no timeline events.
+  double computeStandardDeviationFrameRasterizerTimeMillis() {
+    final List<Duration> durations = _extractGpuRasterizerDrawDurations();
+    final double average = _averageInMillis(durations);
+    double tally = 0.0;
+    for (final Duration duration in durations) {
+      final double time = duration.inMicroseconds.toDouble() / 1000.0;
+      tally += (average - time).abs();
+    }
+    return tally / durations.length;
+  }
+
   /// The longest frame rasterization time in milliseconds.
   ///
   /// Throws a [StateError] if this summary contains no timeline events.
@@ -146,6 +160,9 @@ class TimelineSummary {
   /// * "average_frame_rasterizer_time_millis": Average amount of time spent
   ///   per frame in the engine rasterizer.
   ///   See [computeAverageFrameRasterizerTimeMillis].
+  /// * "stddev_frame_rasterizer_time_millis": Standard deviation of the amount
+  ///   of time spent per frame in the engine rasterizer.
+  ///   See [computeStandardDeviationFrameRasterizerTimeMillis].
   /// * "90th_percentile_frame_rasterizer_time_millis" and
   ///   "99th_percentile_frame_rasterizer_time_millis": The 90/99-th percentile
   ///   frame rasterization time in milliseconds.
@@ -240,6 +257,7 @@ class TimelineSummary {
       'worst_frame_build_time_millis': computeWorstFrameBuildTimeMillis(),
       'missed_frame_build_budget_count': computeMissedFrameBuildBudgetCount(),
       'average_frame_rasterizer_time_millis': computeAverageFrameRasterizerTimeMillis(),
+      'stddev_frame_rasterizer_time_millis': computeStandardDeviationFrameRasterizerTimeMillis(),
       '90th_percentile_frame_rasterizer_time_millis': computePercentileFrameRasterizerTimeMillis(90.0),
       '99th_percentile_frame_rasterizer_time_millis': computePercentileFrameRasterizerTimeMillis(99.0),
       'worst_frame_rasterizer_time_millis': computeWorstFrameRasterizerTimeMillis(),

packages/flutter_driver/test/src/real_tests/timeline_summary_test.dart

@@ -463,6 +463,7 @@ void main() {
             'worst_frame_build_time_millis': 19.0,
             'missed_frame_build_budget_count': 2,
             'average_frame_rasterizer_time_millis': 16.0,
+            'stddev_frame_rasterizer_time_millis': 4.0,
             '90th_percentile_frame_rasterizer_time_millis': 20.0,
             '99th_percentile_frame_rasterizer_time_millis': 20.0,
             'worst_frame_rasterizer_time_millis': 20.0,
@@ -578,6 +579,7 @@ void main() {
           '99th_percentile_frame_build_time_millis': 19.0,
           'missed_frame_build_budget_count': 2,
           'average_frame_rasterizer_time_millis': 16.0,
+          'stddev_frame_rasterizer_time_millis': 4.0,
           '90th_percentile_frame_rasterizer_time_millis': 20.0,
           '99th_percentile_frame_rasterizer_time_millis': 20.0,
           'worst_frame_rasterizer_time_millis': 20.0,