message: json: remove enable_shared_from_this
  I was looking at
  and noticed this. I doubt it's related but we should remove it if
  not used.
diff: @@ -37,7 +37,7 @@ namespace {
  class Field;
  typedef std::shared_ptr<Field> FieldSharedPtr;
  -class Field : public Object, public std::enable_shared_from_this<Field> {
  +class Field : public Object {
  public:
  void setLineNumberStart(uint64_t line_number) { line_number_start_ = line_number; }
  void setLineNumberEnd(uint64_t line_number) { line_number_end_ = line_number; }
  