Add histograms to record the number of writes to the prefs file

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 3bef622b76f2f5612be639532bff71cc3b88394d..9d000341f768a6b6a974e48a879591bb9f84363b 100644
--- a/base/metrics/statistics_recorder.h
+++ b/base/metrics/statistics_recorder.h
@@ -33,6 +33,10 @@ class BASE_EXPORT StatisticsRecorder {
   // Initializes the StatisticsRecorder system. Safe to call multiple times.
   static void Initialize();
 
+  // Reset the StatisticsRecorder system for testing. All existing histogram

[Alexei Svitkine (slow), 2015/04/27 20:21:20]

Nit: Reset -> Resets

To be consistent with the docs for the function above. (Even though I realise
the rest of the file is not consistent.)

[raymes, 2015/04/28 00:32:57]

Done.

+  // data will be erased.
+  static void ResetForTesting();
+
   // Find out if histograms can now be registered into our list.
   static bool IsActive();