* 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
atom::api::WebContents::Type val) {
    using Type = atom::api::WebContents::Type;
    std::string type = "";
    switch (val) {
      case Type::BACKGROUND_PAGE: type = "backgroundPage"; break;
      case Type::BROWSER_WINDOW: type = "window"; break;
      case Type::BROWSER_VIEW: type = "browserView"; break;
      case Type::REMOTE: type = "remote"; break;
      case Type::WEB_VIEW: type = "webview"; break;
      case Type::OFF_SCREEN: type = "offscreen"; break;
      default: break;
    }
    return mate::ConvertToV8(isolate, type);
  }
  static bool FromV8(v8::Isolate* isolate, v8::Local<v8::Value> val,