Add a StatisticsDeltaReader 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 0716e80572d4e314e537a7400e9f8fab294f32e1..5e53f274a0e8ee289b66f908bcea83b8ef6f1362 100644
--- a/base/metrics/statistics_recorder.h
+++ b/base/metrics/statistics_recorder.h
@@ -70,9 +70,9 @@ 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:
@@ -88,6 +88,7 @@ class BASE_EXPORT StatisticsRecorder {
   friend class HistogramBaseTest;
   friend class HistogramTest;
   friend class SparseHistogramTest;
+  friend class StatisticsDeltaReaderTest;
   friend class StatisticsRecorderTest;
   FRIEND_TEST_ALL_PREFIXES(HistogramDeltaSerializationTest,
                            DeserializeHistogramAndAddSamples);