Merge multiple histogram snapshots into single one for reporting.

base/metrics/statistics_recorder.h

Index: base/metrics/statistics_recorder.h
diff --git a/base/metrics/statistics_recorder.h b/base/metrics/statistics_recorder.h
index ef4481abbb4304539a85c4c796136b7e75abbebb..32f381bd717ba82b5a66b995e7beda326f42e402 100644
--- a/base/metrics/statistics_recorder.h
+++ b/base/metrics/statistics_recorder.h
@@ -129,6 +129,10 @@ class BASE_EXPORT StatisticsRecorder {
   // histogram. This method is thread safe.
   static OnSampleCallback FindCallback(const std::string& histogram_name);
 
+  // Clears all of the known histograms and resets static variables to a
+  // state that allows a new initialization.
+  static void ResetForTesting();
+
  private:
   // We keep a map of callbacks to histograms, so that as histograms are
   // created, we can set the callback properly.