[Telemetry] Report stddev for JetStream.

Previously, we flattened all the iterations and reported one number. Now we
report the geo mean of each iteration so that the dashboard can display the
stddev.

BUG=405719

Review URL: https://codereview.chromium.org/493153002

Cr-Commit-Position: refs/heads/master@{#291248}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291248 0039d316-1c4b-4281-b951-d872f2087c98

tools/perf/benchmarks/jetstream.py

@@ -25,7 +25,6 @@
 from telemetry.page import page_test
 from telemetry.util import statistics
 from telemetry.value import list_of_scalar_values
-from telemetry.value import scalar
 
 
 class _JetstreamMeasurement(page_test.PageTest):
@@ -59,16 +58,21 @@ def ValidateAndMeasurePage(self, page, tab, results):
     result = tab.EvaluateJavaScript(get_results_js)
     result = json.loads(result.partition(': ')[2])
 
-    all_scores = []
+    all_score_lists = []
     for k, v in result.iteritems():
       results.AddValue(list_of_scalar_values.ListOfScalarValues(
           results.current_page, k.replace('.', '_'), 'score', v['result'],
           important=False))
       # Collect all test scores to compute geometric mean.
-      all_scores.extend(v['result'])
-    total = statistics.GeometricMean(all_scores)
-    results.AddSummaryValue(
-        scalar.ScalarValue(None, 'Score', 'score', total))
+      for i, score in enumerate(v['result']):
+        if len(all_score_lists) <= i:
+          all_score_lists.append([])
+        all_score_lists[i].append(score)
+    all_scores = []
+    for score_list in all_score_lists:
+      all_scores.append(statistics.GeometricMean(score_list))
+    results.AddSummaryValue(list_of_scalar_values.ListOfScalarValues(
+        None, 'Score', 'score', all_scores))
 
 
 @benchmark.Disabled('android', 'xp')  # crbug.com/381742