Batch pageload metrics pingbacks

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 0a57b30eb7dd3a374f1aee43638ef2084d81f467..595da0dc864ea328f3fc588fa0c57c509c2d60eb 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -7767,31 +7767,32 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>
     Counts the number of times various events occur when the data reduction
     proxy is enabled and the IP address of the client changes.
   </summary>
 </histogram>
 
 <histogram name="DataReductionProxy.Pingback.Attempted" enum="BooleanAttempted">
   <owner>bengr@chromium.org</owner>
   <owner>ryansturm@chromium.org</owner>
   <summary>
-    Counts the number of times a pingback was attempted or not attempted based
-    on the reporting fraction.
+    Counts the number of times that pageload metrics were queued to be attempted
+    or not queued based on the reporting fraction. Recorded once per pageload.
   </summary>
 </histogram>
 
 <histogram name="DataReductionProxy.Pingback.Succeeded" enum="BooleanSuccess">
   <owner>bengr@chromium.org</owner>
   <owner>ryansturm@chromium.org</owner>
   <summary>
-    Counts the number of times that a pingback succeeded or failed at being sent
-    to the server.
+    Counts the number of times that a batched pingback request succeeded or
+    failed at being sent to the server. Recorded everytime a pingback request is
+    attempted.
   </summary>
 </histogram>
 
 <histogram name="DataReductionProxy.ProbeURL"
     enum="DataReductionProxyProbeURLFetchResult">
   <owner>bengr@chromium.org</owner>
   <owner>marq@chromium.org</owner>
   <summary>
     Counts various outcomes of requesting the data reduction proxy's probe URL.
   </summary>