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..fded3f289ff9dc959744b8701fa186ccc005bc8d 100644
--- a/base/json/json_file_value_serializer.h
+++ b/base/json/json_file_value_serializer.h
@@ -14,15 +14,21 @@
 
 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) {}
+  // If |histogram_suffix| is non-empty, will log a
+  // "Settings.JsonDataSizeKilobytes.|histogram_suffix|" UMA histogram with the
+  // size of the content read from disk while deserializing.
+  JSONFileValueSerializer(const base::FilePath& json_file_path,
+                          const std::string& histogram_suffix);

[Nico, 2015/01/30 18:46:28]

Why does a json library need to expose histograms in its public api?

Even the previous histogram code in the json implementation looks pretty fishy
to me (I hadn't seen it before), but at least it was some internal thing.

 
-  ~JSONFileValueSerializer() override {}
+  // Constructs a JSONFileValueSerializer as above but with an empty
+  // |histogram_suffix|.
+  explicit JSONFileValueSerializer(const base::FilePath& json_file_path);
+
+  ~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
@@ -74,8 +80,9 @@ class BASE_EXPORT JSONFileValueSerializer : public base::ValueSerializer {
  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_;
+  const std::string histogram_suffix_;
 
   // A wrapper for ReadFileToString which returns a non-zero JsonFileError if
   // there were file errors.