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 b42135ab4be8d99939fdea5df70e578b95f3b3bb..35045b94daae89a0786418e6d544d2e9a06f9145 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -69812,6 +69812,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="UMA.NegativeSamples.Reason" enum="NegativeSampleReason">
+  <owner>asvitkine@chromium.org</owner>
+  <owner>bcwhite@chromium.org</owner>
+  <summary>
+    Reasons why a negative sample count would have been created.

[Alexei Svitkine (slow), 2017/01/20 19:11:30]

This is for sparse histograms specifically, right? If so, mention it in the
description.

[bcwhite, 2017/01/20 19:17:06]

Currently there are only sparse sources but I worded it this way on the
assumption that it may be necessary to expands its scope.

[Alexei Svitkine (slow), 2017/01/20 19:21:30]

OK. Can you add a note about it being currently sparse though? The note can be
removed/modified in the future if we add more support.

[bcwhite, 2017/01/20 19:59:19]

Done.

+  </summary>
+</histogram>
+
 <histogram name="UMA.Perf.GetData" enum="GetPerfDataOutcome">
   <owner>asvitkine@chromium.org</owner>
   <summary>
@@ -97171,6 +97179,16 @@ value.
   <int value="5" label="Active network was unblocked"/>
 </enum>
 
+<enum name="NegativeSampleReason" type="int">
+  <int value="0" label="Delta was not negative."/>
+  <int value="1"
+      label="Persistent sparse histogram had logged value but no active
+             sample."/>
+  <int value="2"
+      label="Persistent sparse histogram active sample less than logged
+             value."/>
+</enum>
+
 <enum name="NetCacheState" type="int">
   <int value="0" label="FROM_CACHE"/>
   <int value="1" label="STILL_VALID"/>