* 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_window.cc
auto* existing_preferences =
        WebContentsPreferences::FromWebContents(web_contents->web_contents());
    base::DictionaryValue web_preferences_dict;
    if (mate::ConvertFromV8(isolate, web_preferences.GetHandle(),
                        &web_preferences_dict)) {
      existing_preferences->web_preferences()->Clear();
      existing_preferences->Merge(web_preferences_dict);
    }
  } else {
    
    web_contents = WebContents::Create(isolate, web_preferences);
  }
  Init(isolate, wrapper, options, web_contents);
}
void Window::Init(v8::Isolate* isolate,
                  v8::Local<v8::Object> wrapper,
                  const mate::Dictionary& options,
                  mate::Handle<class WebContents> web_contents) {
  web_contents_.Reset(isolate, web_contents.ToV8());
  api_web_contents_ = web_contents.get();
  
  mate::Dictionary(isolate, web_contents->GetWrapper()).Set(
      "browserWindowOptions", options);
  
  mate::Handle<Window> parent;
  if (options.Get("parent", &parent))
    parent_window_.Reset(isolate, parent.ToV8());