PageLoadMetrics renderer and browser implementation.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index e9b82d75dbb11aafe93d7af740f87cd277ce45d4..05daec71bcf6afe8daf617fd45d01b466ee0b244 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -29367,6 +29367,31 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PageLoad.Timing.DOMContentLoadedEventFired"
+    units="milliseconds">
+  <owner>bmcquade@chromium.org</owner>
+  <summary>
+    Measures the time from navigation timing's navigation start to the time the
+    DOMContentLoaded event is fired, for main frame documents.
+  </summary>
+</histogram>
+
+<histogram name="PageLoad.Timing.FirstLayout" units="milliseconds">
+  <owner>bmcquade@chromium.org</owner>
+  <summary>
+    Measures the time from navigation timing's navigation start to the time the
+    first layout is performed, for main frame documents.
+  </summary>
+</histogram>
+
+<histogram name="PageLoad.Timing.LoadEventFired" units="milliseconds">
+  <owner>bmcquade@chromium.org</owner>
+  <summary>
+    Measures the time from navigation timing's navigation start to the time the
+    load event is fired, for main frame documents.
+  </summary>
+</histogram>
+
 <histogram name="PartitionAlloc.CommittedSize" units="MB">
   <owner>haraken@chromium.org</owner>
   <summary>