Telemetry: Check in PrintPerfResult that the flattened list is also non-zero.

tools/telemetry/telemetry/page/buildbot_page_measurement_results.py

Index: tools/telemetry/telemetry/page/buildbot_page_measurement_results.py
diff --git a/tools/telemetry/telemetry/page/buildbot_page_measurement_results.py b/tools/telemetry/telemetry/page/buildbot_page_measurement_results.py
index 434b8147567b777d48eb30b97d5880aad21df3c7..73a58aa732cd7d19a63e52be220fe35b6abbd9cf 100644
--- a/tools/telemetry/telemetry/page/buildbot_page_measurement_results.py
+++ b/tools/telemetry/telemetry/page/buildbot_page_measurement_results.py
@@ -89,10 +89,11 @@ class BuildbotPageMeasurementResults(
             continue
           name_value_map[name].append(value)
         for name in unique_pages:
-          if not len(name_value_map[name]):
+          values = perf_tests_helper.FlattenList(name_value_map[name])
+          if not len(values):
             continue
           self._PrintPerfResult(measurement + '_by_url', name,
-                                name_value_map[name], units, by_name_data_type)
+                                values, units, by_name_data_type)
 
       # If there were no page failures, print the average data.
       # For histograms, we don't print the average data, only the _by_name,