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

base/prefs/json_pref_store.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_PREFS_JSON_PREF_STORE_H_
 #define BASE_PREFS_JSON_PREF_STORE_H_
 
 #include <set>
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/callback_forward.h"
 #include "base/compiler_specific.h"
 #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"
 #include "base/prefs/base_prefs_export.h"
 #include "base/prefs/persistent_pref_store.h"
 #include "base/threading/non_thread_safe.h"
 
 class PrefFilter;
 
 namespace base {
@@ skipping 64 matching lines @@
   // Just like RemoveValue(), but doesn't notify observers. Used when doing some
   // cleanup that shouldn't otherwise alert observers.
   void RemoveValueSilently(const std::string& key);
 
   // Registers |on_next_successful_write| to be called once, on the next
   // successful write event of |writer_|.
   void RegisterOnNextSuccessfulWriteCallback(
       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
   // |value| (or an empty dictionary in some read error cases) to the
   // |pref_filter| if one is set. It also gives a callback pointing at
   // FinalizeFileRead() to that |pref_filter_| which is then responsible for
   // invoking it when done. If there is no |pref_filter_|, FinalizeFileRead()
   // is invoked directly.
   void OnFileRead(scoped_ptr<ReadResult> read_result);
 
@@ skipping 26 matching lines @@
   ObserverList<PrefStore::Observer, true> observers_;
 
   scoped_ptr<ReadErrorDelegate> error_delegate_;
 
   bool initialized_;
   bool filtering_in_progress_;
   PrefReadError read_error_;
 
   std::set<std::string> keys_need_empty_value_;
 
+  scoped_ptr<WriteCountHistogram> write_count_histogram_;
+
   DISALLOW_COPY_AND_ASSIGN(JsonPrefStore);
 };
 
 #endif  // BASE_PREFS_JSON_PREF_STORE_H_