Add support for base histograms in histograms.xml.

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 9d00901a8d18cb0f51b705b2daf771e47e753501..76f3f68ff5c93fb5bc84293a4ea29357cfa9fb2b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -40191,7 +40191,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
-<histogram name="PageLoad.AbortTiming.Background" units="ms">
+<histogram base="true" name="PageLoad.AbortTiming.Background" units="ms">
   <owner>bmcquade@chromium.org</owner>
   <owner>csharrison@chromium.org</owner>
   <summary>
@@ -40201,7 +40201,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
-<histogram name="PageLoad.AbortTiming.ClientRedirect" units="ms">
+<histogram base="true" name="PageLoad.AbortTiming.ClientRedirect" units="ms">
   <owner>csharrison@chromium.org</owner>
   <summary>
     This metric is still experimental and not yet ready to be relied upon.
@@ -40210,7 +40210,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
-<histogram name="PageLoad.AbortTiming.Close" units="ms">
+<histogram base="true" name="PageLoad.AbortTiming.Close" units="ms">
   <owner>csharrison@chromium.org</owner>
   <summary>
     This metric is still experimental and not yet ready to be relied upon.
@@ -40219,7 +40219,8 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
-<histogram name="PageLoad.AbortTiming.ForwardBackNavigation" units="ms">
+<histogram base="true" name="PageLoad.AbortTiming.ForwardBackNavigation"
+    units="ms">
   <owner>csharrison@chromium.org</owner>
   <summary>
     This metric is still experimental and not yet ready to be relied upon.
@@ -40228,7 +40229,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
-<histogram name="PageLoad.AbortTiming.NewNavigation" units="ms">
+<histogram base="true" name="PageLoad.AbortTiming.NewNavigation" units="ms">
   <owner>csharrison@chromium.org</owner>
   <summary>
     This metric is still experimental and not yet ready to be relied upon.
@@ -40237,7 +40238,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
-<histogram name="PageLoad.AbortTiming.Other" units="ms">
+<histogram base="true" name="PageLoad.AbortTiming.Other" units="ms">
   <owner>csharrison@chromium.org</owner>
   <summary>
     This metric is still experimental and not yet ready to be relied upon.
@@ -40246,7 +40247,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
-<histogram name="PageLoad.AbortTiming.Reload" units="ms">
+<histogram base="true" name="PageLoad.AbortTiming.Reload" units="ms">
   <owner>csharrison@chromium.org</owner>
   <summary>
     This metric is still experimental and not yet ready to be relied upon.
@@ -40255,7 +40256,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
-<histogram name="PageLoad.AbortTiming.Stop" units="ms">
+<histogram base="true" name="PageLoad.AbortTiming.Stop" units="ms">
   <owner>csharrison@chromium.org</owner>
   <summary>
     This metric is still experimental and not yet ready to be relied upon.
@@ -107988,8 +107989,8 @@ value.
 <histogram_suffixes name="PageLoadEventConditions" separator=".">
   <suffix name="BeforeCommit"/>
   <suffix name="AfterCommit.BeforePaint"/>
-  <suffix name="AfterPaint.BeforeInteraction"/>
-  <suffix name="AfterPaint.Before1sDelayedInteraction"/>
+  <suffix base="true" name="AfterPaint.BeforeInteraction"/>
+  <suffix base="true" name="AfterPaint.Before1sDelayedInteraction"/>
   <suffix name="DuringParse"/>
   <affected-histogram name="PageLoad.AbortTiming.Background"/>
   <affected-histogram name="PageLoad.AbortTiming.ClientRedirect"/>