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 afd93fb6f832f9186c35e3cfdb7f17cf953edf8b..0baf09dd0bb712275df99f09a384aa749b524eb2 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -29643,6 +29643,32 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PageLoad.Timing.NavigationToDOMContentLoadedEventFired"
+    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.NavigationToFirstLayout" 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.NavigationToLoadEventFired"
+    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>