Add a StatisticsDeltaReader class and use cases

This CL adds a utility class to streamline the task of testing that
metrics have been updated as expected. Specifically, the
StatisticsDeltaReader class allows a client to obtain the differential
value of a given histogram in the interval since the given
StatisticsDeltaReader instance was created.

This reverts commit 6a7dfb3cd638829a04e002ea9bc5b18ed3c9c03f.

BUG=232414
TBR=rsleevy, groby

Review URL: https://codereview.chromium.org/120753002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243303 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 4 insertions, 3 deletions

diff --git a/base/metrics/statistics_recorder.h b/base/metrics/statistics_recorder.h
index 0716e80..5e53f274 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);