Separate page load metrics for backgrounded pages

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 601cee84d75b23938d0d865e3d9ce336c33ce0f6..42e8c23ce0be86a014d55b29ea4f1fed92dbe266 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -29710,6 +29710,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 <histogram name="PageLoad.Timing.NavigationToDOMContentLoadedEventFired"
     units="milliseconds">
   <owner>bmcquade@chromium.org</owner>
+  <owner>csharrison@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.
@@ -29718,6 +29719,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 
 <histogram name="PageLoad.Timing.NavigationToFirstLayout" units="milliseconds">
   <owner>bmcquade@chromium.org</owner>
+  <owner>csharrison@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.
@@ -29727,6 +29729,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 <histogram name="PageLoad.Timing.NavigationToLoadEventFired"
     units="milliseconds">
   <owner>bmcquade@chromium.org</owner>
+  <owner>csharrison@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.
@@ -77141,6 +77144,15 @@ To add a new entry, add it with any value and run test to compute valid value.
   <affected-histogram name="PLT.LoadType"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="PageLoadBackgrounded" separator=".">
+  <suffix name="BG"
+      label="Event occurred at least partially in the background"/>
+  <affected-histogram
+      name="PageLoad.Timing.NavigationToDOMContentLoadedEventFired"/>
+  <affected-histogram name="PageLoad.Timing.NavigationToFirstLayout"/>
+  <affected-histogram name="PageLoad.Timing.NavigationToLoadEventFired"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="PageLoadType">
   <suffix name="HistoryLoad"
       label="but only for user pressing back or forward"/>