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

base/json/json_file_value_serializer.cc

 // 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.
 
 #include "base/json/json_file_value_serializer.h"
 
 #include "base/files/file_util.h"
 #include "base/json/json_string_value_serializer.h"
 #include "base/logging.h"
 
 using base::FilePath;
 
 const char JSONFileValueSerializer::kAccessDenied[] = "Access denied.";
 const char JSONFileValueSerializer::kCannotReadFile[] = "Can't read file.";
 const char JSONFileValueSerializer::kFileLocked[] = "File locked.";
 const char JSONFileValueSerializer::kNoSuchFile[] = "File doesn't exist.";
 
+JSONFileValueSerializer::JSONFileValueSerializer(
+    const base::FilePath& json_file_path)
+    : json_file_path_(json_file_path),
+      allow_trailing_comma_(false),
+      last_read_size_(0U) {}
+
+JSONFileValueSerializer::~JSONFileValueSerializer() {}
+
 bool JSONFileValueSerializer::Serialize(const base::Value& root) {
   return SerializeInternal(root, false);
 }
 
 bool JSONFileValueSerializer::SerializeAndOmitBinaryValues(
     const base::Value& root) {
   return SerializeInternal(root, true);
 }
 
 bool JSONFileValueSerializer::SerializeInternal(const base::Value& root,
@@ skipping 24 matching lines @@
       return JSON_FILE_LOCKED;
     } else if (error == ERROR_ACCESS_DENIED) {
       return JSON_ACCESS_DENIED;
     }
 #endif
     if (!base::PathExists(json_file_path_))
       return JSON_NO_SUCH_FILE;
     else
       return JSON_CANNOT_READ_FILE;
   }
+
+  last_read_size_ = json_string->size();
   return JSON_NO_ERROR;
 }
 
 const char* JSONFileValueSerializer::GetErrorMessageForCode(int error_code) {
   switch (error_code) {
     case JSON_NO_ERROR:
       return "";
     case JSON_ACCESS_DENIED:
       return kAccessDenied;
     case JSON_CANNOT_READ_FILE:
@@ skipping 17 matching lines @@
       *error_code = error;
     if (error_str)
       *error_str = GetErrorMessageForCode(error);
     return NULL;
   }
 
   JSONStringValueSerializer serializer(json_string);
   serializer.set_allow_trailing_comma(allow_trailing_comma_);
   return serializer.Deserialize(error_code, error_str);
 }