[UMA, Cleanup] Deprecate the UMA.GeneratedLowEntropySource histogram.

This histogram is no longer being tracked.

BUG=517206
TEST=none
R=asvitkine@chromium.org

Review URL: https://codereview.chromium.org/1284433003

Cr-Commit-Position: refs/heads/master@{#342469}
diff --git a/components/metrics/metrics_state_manager.h b/components/metrics/metrics_state_manager.h
index 041831c..fd80c235 100644
--- a/components/metrics/metrics_state_manager.h
+++ b/components/metrics/metrics_state_manager.h
@@ -117,6 +117,10 @@
   // generate the entropy source value if it has not been called before.
   int GetLowEntropySource();
 
+  // Generates the low entropy source value for this client if it is not
+  // already set.
+  void UpdateLowEntropySource();
+
   // 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);