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

base/json/json_file_value_serializer.h

Index: base/json/json_file_value_serializer.h
diff --git a/base/json/json_file_value_serializer.h b/base/json/json_file_value_serializer.h
index f0f556c2dc68a23457ad2aa8ec3db7a8a209947d..6cfcbe83c2f8295fcd5f277bf637b6afda809e2a 100644
--- a/base/json/json_file_value_serializer.h
+++ b/base/json/json_file_value_serializer.h
@@ -14,15 +14,13 @@
 
 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
@@ -71,11 +69,16 @@ class BASE_EXPORT JSONFileValueSerializer : public base::ValueSerializer {
     allow_trailing_comma_ = new_value;
   }
 
+  // Returns the syze (in bytes) of JSON string read from disk in the last
+  // successful |Deserialize()| call.
+  size_t get_last_read_size() const { return last_read_size_; }
+
  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.