* Electron version: 1.6.15
* Operating system: Linux

### Expected behavior

Open a `BrowserWindow` with passed options

### Actual behavior

  - Usually a Segmentation fault.
  - Sometimes `[12843:0100/000000.043847:FATAL:zygote_linux.cc(479)] Failed to synchronise with parent zygote process`
  - Sometimes `[12819:0100/000000.719122:FATAL:zygote_linux.cc(509)] Check failed: len > 0 (0 vs. 0)`

### How to reproduce

I'm still working on making a smaller reproduction case. The issue goes away with small tweaks.

1. Starting from https://github.com/daurnimator/fengari-electron-example
2. Modify main.lua to contain:
```lua
local js = require "js"
local electron = require "electron"

electron.app:on("ready", function()
	local win = js.new(electron.BrowserWindow, js.createproxy({
		backgroundColor = '#2e2c29';
	}, "object"))
	win:loadURL("https://fengari.io")
end)
```

atom/browser/api/atom_api_web_contents.cc
PrintSettings* out) {
    mate::Dictionary dict;
    if (!ConvertFromV8(isolate, val, &dict))
      return false;
    dict.Get("silent", &(out->silent));
    dict.Get("printBackground", &(out->print_background));
    dict.Get("deviceName", &(out->device_name));
    return true;
  }
};
template<>
struct Converter<printing::PrinterBasicInfo> {
  static v8::Local<v8::Value> ToV8(v8::Isolate* isolate,
                                   const printing::PrinterBasicInfo& val) {
    mate::Dictionary dict(isolate, v8::Object::New(isolate));
    dict.Set("name", val.printer_name);
    dict.Set("description", val.printer_description);
    dict.Set("status", val.printer_status);
    dict.Set("isDefault", val.is_default ? true : false);
    dict.Set("options", val.options);
    return dict.GetHandle();
  }
};
template<>
struct Converter<WindowOpenDisposition> {
  static v8::Local<v8::Value> ToV8(v8::Isolate* isolate,