Add a histogram to debug metrics entropy source use.

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 6173e3de0485f977d8c524ef490082660b9e99c7..d91f64c5f4dc68023a557a19983cbce5f879a7ec 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -39989,6 +39989,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="UMA.EntropySourceType" enum="UmaEntropySourceType">
+  <owner>asvitkine@chromium.org</owner>
+  <summary>
+    Logs the entropy source type that was used for field trial randomization.
+    Logged once at each start up.
+  </summary>
+</histogram>
+
 <histogram name="UMA.FieldTrialsEnabledBenchmarking" enum="BooleanUsage">
   <obsolete>
     Deprecated 2012. No longer tracked.
@@ -59968,6 +59976,12 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="5" label="Missing/Clean (Registry/Local State)"/>
 </enum>
 
+<enum name="UmaEntropySourceType" type="int">
+  <int value="0" label="No entropy source (never hit)"/>
+  <int value="1" label="Low Entropy Source"/>
+  <int value="2" label="High Entropy Source"/>
+</enum>
+
 <enum name="UmaInitSequence" type="int">
   <int value="0" label="Timer fired first"/>
   <int value="1" label="Init task completed first"/>