Create and display "comparison statistics" to compare two statistics

tools/telemetry/telemetry/page/page_measurement_results.py

Index: tools/telemetry/telemetry/page/page_measurement_results.py
diff --git a/tools/telemetry/telemetry/page/page_measurement_results.py b/tools/telemetry/telemetry/page/page_measurement_results.py
index 02ebd2a03b48bfebecc360996074fedc6a0043c3..07a09a9c0ba0c84d07defe4edfb1b27a81e66a8e 100644
--- a/tools/telemetry/telemetry/page/page_measurement_results.py
+++ b/tools/telemetry/telemetry/page/page_measurement_results.py
@@ -50,6 +50,8 @@ class PageMeasurementResults(page_test_results.PageTestResults):
 
     self._values_for_current_page = {}
 
+    self._comparison_stats = {}
+
   def __getitem__(self, i):
     """Shorthand for self.page_results[i]"""
     return self._page_results[i]
@@ -83,6 +85,18 @@ class PageMeasurementResults(page_test_results.PageTestResults):
                                         data_type)
     self._overall_results.append(value)
 
+  def AddComparison(self, reference_trace_name, reference_chart_name,
+                    comparison_trace_name, comparison_chart_name,
+                    result_trace_name, result_chart_name):
+    """Specify a new statistic to be generated from the comparison between
+    two existing statistics when statistics are analyzed."""
+    self._comparison_stats[page_measurement_value.MeasurementName(
+        result_chart_name, result_trace_name)] = {
+      'reference' : page_measurement_value.MeasurementName(
+        reference_chart_name, reference_trace_name),
+      'comparison' : page_measurement_value.MeasurementName(
+        comparison_chart_name, comparison_trace_name) }
+
   def _GetPageMeasurementValue(self, trace_name, units, value, chart_name,
                              data_type):
     value = page_measurement_value.PageMeasurementValue(