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

base/metrics/statistics_recorder.cc

Index: base/metrics/statistics_recorder.cc
diff --git a/base/metrics/statistics_recorder.cc b/base/metrics/statistics_recorder.cc
index 23c28d4943b4e1b28006a43ae050eb03a3a00ff0..81f288055d86c515e6d0ca215032da3fed28dad3 100644
--- a/base/metrics/statistics_recorder.cc
+++ b/base/metrics/statistics_recorder.cc
@@ -31,6 +31,12 @@ void StatisticsRecorder::Initialize() {
   g_statistics_recorder_.Get();
 }
 
+// static
+void StatisticsRecorder::ResetForTesting() {
+  Initialize();
+  histograms_->clear();
+  ranges_->clear();
+}
 
 // static
 bool StatisticsRecorder::IsActive() {