Record the time to requestStart from the javascript timings API as part of the startup benchmark.

tools/perf/metrics/startup_metric.py

Index: tools/perf/metrics/startup_metric.py
diff --git a/tools/perf/metrics/startup_metric.py b/tools/perf/metrics/startup_metric.py
index 6e6298d702de4f516b7fd95d0346d9ac1d28af40..af05c8f89aec401c83a3407ff4ef6bf944c74061 100644
--- a/tools/perf/metrics/startup_metric.py
+++ b/tools/perf/metrics/startup_metric.py
@@ -47,7 +47,7 @@ class StartupMetric(Metric):
     tab_load_times = []
     TabLoadTime = collections.namedtuple(
         'TabLoadTime',
-        ['load_start_ms', 'load_duration_ms'])
+        ['load_start_ms', 'load_duration_ms', 'request_start_ms'])
 
     def RecordTabLoadTime(t):
       try:
@@ -65,9 +65,14 @@ class StartupMetric(Metric):
           print "Page: ", tab_title, " didn't finish loading."
           return
 
+        perf_timing = t.EvaluateJavaScript('window.performance.timing')
+        if 'requestStart' not in perf_timing:
+          raise Exception("requestStart not in window performance timings")

[pasko, 2014/09/17 13:25:08]

this would abort execution of all tests. Even if all browsers able to run with
telemetry have requestStart, still scary.

[azarchs, 2014/09/17 13:57:16]

Done.

+
         tab_load_times.append(TabLoadTime(
             int(result['load_start_ms']),
-            int(result['load_duration_ms'])))
+            int(result['load_duration_ms']),
+            int(perf_timing['requestStart'])))
       except util.TimeoutException:
         # Low memory Android devices may not be able to load more than
         # one tab at a time, so may timeout when the test attempts to
@@ -88,6 +93,10 @@ class StartupMetric(Metric):
     results.AddValue(scalar.ScalarValue(
         results.current_page, 'foreground_tab_load_complete', 'ms',
         foreground_tab_load_complete))
+    if (foreground_tab_stats.request_start_ms > 0):
+      results.AddValue(scalar.ScalarValue(
+          results.current_page, 'foreground_tab_request_start', 'ms',

[pasko, 2014/09/17 13:25:08]

why absolute value and not the relative one? LoadingMetric reports it as
relative to navigationStart. I am not sure in which circumstances we use each
metric though.

[azarchs, 2014/09/17 13:57:16]

The value reported here is relative to the process start. 
window.performance.timing.requestStart is an absolute time from 1970.

+          foreground_tab_stats.request_start_ms - browser_main_entry_time_ms))
 
   def AddResults(self, tab, results):
     get_histogram_js = 'statsCollectionController.getBrowserHistogram("%s")'