Log pref file size histogram on read rather than on write.

base/json/json_file_value_serializer.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_JSON_JSON_FILE_VALUE_SERIALIZER_H_
 #define BASE_JSON_JSON_FILE_VALUE_SERIALIZER_H_
 
 #include <string>
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/files/file_path.h"
 #include "base/values.h"
 
 class BASE_EXPORT JSONFileValueSerializer : public base::ValueSerializer {
  public:
-  // json_file_patch is the path of a file that will be source of the
+  // |json_file_path_| is the path of a file that will be source of the
   // deserialization or the destination of the serialization.
   // When deserializing, the file should exist, but when serializing, the
   // serializer will attempt to create the file at the specified location.
-  explicit JSONFileValueSerializer(const base::FilePath& json_file_path)
-    : json_file_path_(json_file_path),
-      allow_trailing_comma_(false) {}
+  explicit JSONFileValueSerializer(const base::FilePath& json_file_path);
 
-  ~JSONFileValueSerializer() override {}
+  ~JSONFileValueSerializer() override;
 
   // DO NOT USE except in unit tests to verify the file was written properly.
   // We should never serialize directly to a file since this will block the
   // thread. Instead, serialize to a string and write to the file you want on
   // the file thread.
   //
   // Attempt to serialize the data structure represented by Value into
   // JSON.  If the return value is true, the result will have been written
   // into the file whose name was passed into the constructor.
   bool Serialize(const base::Value& root) override;
@@ skipping 28 matching lines @@
   static const char kNoSuchFile[];
 
   // Convert an error code into an error message.  |error_code| is assumed to
   // be a JsonFileError.
   static const char* GetErrorMessageForCode(int error_code);
 
   void set_allow_trailing_comma(bool new_value) {
     allow_trailing_comma_ = new_value;
   }
 
+  // Returns the syze (in bytes) of JSON string read from disk in the last
+  // |Deserialize()| call.

[Alexei Svitkine (slow), 2015/01/30 20:46:32]

Looks like you're only updating the field if ReadFileToString() is successful.
Please update the comment to mention this.

Also, maybe only log the histogram on success too..

[gab, 2015/01/31 16:46:56]

Done.

+  size_t get_last_read_size() { return last_read_size_; }

[Alexei Svitkine (slow), 2015/01/30 20:46:32]

Nit: const

[gab, 2015/01/31 16:46:56]

Done.

+
  private:
   bool SerializeInternal(const base::Value& root, bool omit_binary_values);
 
-  base::FilePath json_file_path_;
+  const base::FilePath json_file_path_;
   bool allow_trailing_comma_;
+  size_t last_read_size_;
 
   // A wrapper for ReadFileToString which returns a non-zero JsonFileError if
   // there were file errors.
   int ReadFileToString(std::string* json_string);
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(JSONFileValueSerializer);
 };
 
 #endif  // BASE_JSON_JSON_FILE_VALUE_SERIALIZER_H_