When specifying a new create histogram callback, reset all histograms so they recreate themselves o…

src/counters.h

Index: src/counters.h
diff --git a/src/counters.h b/src/counters.h
index 233837635a411261d051f17c4a77b5e40a7e20dd..577280f444848a7511da42d91f7fe229c0569910 100644
--- a/src/counters.h
+++ b/src/counters.h
@@ -219,6 +219,11 @@ struct Histogram {
     return GetHistogram() != NULL;
   }
 
+  // Reset the cached internal pointer.
+  void Reset() {
+    lookup_done_ = false;
+  }
+
  protected:
   // Returns the handle to the histogram.
   void* GetHistogram() {
@@ -251,6 +256,10 @@ struct HistogramTimer {
   bool Running() {
     return histogram_.Enabled() && (start_time_ != 0) && (stop_time_ == 0);
   }
+
+  void Reset() {
+    histogram_.Reset();
+  }
 };
 
 // Helper class for scoping a HistogramTimer.