Report reasons for a negative histogram sample count.

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 3ae6ea6b64e40cd50c7bcd54b983ef12f3c010d9..1a4dff3bc1f24a1b988b16d082cda71453bbe12f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -70599,6 +70599,15 @@ 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. Currently, only
+    negative samples from persistent sparse histograms are being checked.
+  </summary>
+</histogram>
+
 <histogram name="UMA.Perf.GetData" enum="GetPerfDataOutcome">
   <owner>asvitkine@chromium.org</owner>
   <summary>
@@ -97944,6 +97953,15 @@ value.
   <int value="5" label="Active network was unblocked"/>
 </enum>
 
+<enum name="NegativeSampleReason" type="int">
+  <int value="0"
+      label="Persistent sparse histogram had logged value but no active
+             sample."/>
+  <int value="1"
+      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"/>