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 1b79ca737d73b284ba3c4f9ff1e2fa782e539d38..bee84942839440dfc8c090d358ea10a71a6cb383 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -40306,7 +40306,7 @@ 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>
@@ -40316,7 +40316,7 @@ 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.
@@ -40325,7 +40325,7 @@ 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.
@@ -40334,7 +40334,8 @@ 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.
@@ -40343,7 +40344,7 @@ 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.
@@ -40352,7 +40353,7 @@ 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.
@@ -40361,7 +40362,7 @@ 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.
@@ -40370,7 +40371,7 @@ 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.
@@ -108285,8 +108286,8 @@ 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"/>