Add page load abort metrics to PageLoadMetrics system

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 aef42664ff3142f49aab78440a55ba96fdd474e3..e05edc5bc732617912f3bcade14f33896117d2a4 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -29783,6 +29783,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PageLoad.EventCounts" enum="PageLoadEvent">
+  <owner>csharrison@chromium.org</owner>
+  <owner>bmcquade@chromium.org</owner>
+  <summary>
+    Captures counts of various page load events. These are enumerated in the
+    enum page_load_metrics::PageLoadEvent, and include events like 'page load
+    aborted before first layout'
+  </summary>
+</histogram>
+
 <histogram name="PageLoad.Timing.NavigationToDOMContentLoadedEventFired"
     units="milliseconds">
   <owner>bmcquade@chromium.org</owner>
@@ -66515,6 +66525,15 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="4" label="Index"/>
 </enum>
 
+<enum name="PageLoadEvent" type="int">
+  <int value="0" label="Started main frame load"/>
+  <int value="1" label="Failed load before commit"/>
+  <int value="2" label="Aborted load before commit"/>
+  <int value="3" label="Aborted load before first layout"/>
+  <int value="4" label="Successful first layout (foreground)"/>
+  <int value="5" label="Successful first layout (backgrounded)"/>
+</enum>
+
 <enum name="PagespeedHeaderServerType" type="int">
   <int value="0" label="Total responses"/>
   <int value="1" label="mod_pagespeed server"/>