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

base/prefs/json_pref_store.h

Index: base/prefs/json_pref_store.h
diff --git a/base/prefs/json_pref_store.h b/base/prefs/json_pref_store.h
index 16e431b3c92f014cbf57afc766710f629d23995c..c20b109c62258ca46293d006710c16b374c7f0da 100644
--- a/base/prefs/json_pref_store.h
+++ b/base/prefs/json_pref_store.h
@@ -14,6 +14,7 @@
 #include "base/files/file_path.h"
 #include "base/files/important_file_writer.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/memory/scoped_vector.h"
 #include "base/memory/weak_ptr.h"
 #include "base/message_loop/message_loop_proxy.h"
 #include "base/observer_list.h"
@@ -98,6 +99,8 @@ class BASE_PREFS_EXPORT JsonPrefStore
       const base::Closure& on_next_successful_write);
 
  private:
+  class WriteCountHistogram;
+
   ~JsonPrefStore() override;
 
   // This method is called after the JSON file has been read.  It then hands
@@ -144,6 +147,8 @@ class BASE_PREFS_EXPORT JsonPrefStore
 
   std::set<std::string> keys_need_empty_value_;
 
+  scoped_ptr<WriteCountHistogram> write_count_histogram_;
+
   DISALLOW_COPY_AND_ASSIGN(JsonPrefStore);
 };