Add a histogram to debug metrics entropy source use.

components/metrics/metrics_state_manager.h

Index: components/metrics/metrics_state_manager.h
diff --git a/components/metrics/metrics_state_manager.h b/components/metrics/metrics_state_manager.h
index 30b6285a6a3f1e8efa2ffd3e420ff75120355ad1..041831c52fbbf0587b51427d210ef8e962909980 100644
--- a/components/metrics/metrics_state_manager.h
+++ b/components/metrics/metrics_state_manager.h
@@ -90,6 +90,7 @@ class MetricsStateManager {
     ENTROPY_SOURCE_NONE,
     ENTROPY_SOURCE_LOW,
     ENTROPY_SOURCE_HIGH,
+    ENTROPY_SOURCE_ENUM_SIZE,
   };
 
   // Creates the MetricsStateManager with the given |local_state|. Calls
@@ -116,6 +117,10 @@ class MetricsStateManager {
   // generate the entropy source value if it has not been called before.
   int GetLowEntropySource();
 
+  // Updates |entropy_source_returned_| with |type| iff the current value is
+  // ENTROPY_SOURCE_NONE and logs the new value in a histogram.
+  void UpdateEntropySourceReturnedValue(EntropySourceType type);
+
   // Returns the first entropy source that was returned by this service since
   // start up, or NONE if neither was returned yet. This is exposed for testing
   // only.