Add a HistogramRecorder class and use cases.

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 9a5522532439e256d0f3dd2d59319bd0c9230cee..744be0110b6d3ef7adaad38595051656b72ccbd4 100644
--- a/base/metrics/statistics_recorder.h
+++ b/base/metrics/statistics_recorder.h
@@ -66,9 +66,9 @@ class BASE_EXPORT StatisticsRecorder {
   static HistogramBase* FindHistogram(const std::string& name);
 
   // GetSnapshot copies some of the pointers to registered histograms into the
-  // caller supplied vector (Histograms).  Only histograms with names matching
-  // query are returned. The query must be a substring of histogram name for its
-  // pointer to be copied.
+  // caller supplied vector (Histograms).  Only histograms which have query as
+  // a substring are copied (an empty string will process all registered
+  // histograms).
   static void GetSnapshot(const std::string& query, Histograms* snapshot);
 
  private: