* 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
WindowOpenDisposition val) {
    std::string disposition = "other";
    switch (val) {
      case WindowOpenDisposition::CURRENT_TAB:
        disposition = "default";
        break;
      case WindowOpenDisposition::NEW_FOREGROUND_TAB:
        disposition = "foreground-tab";
        break;
      case WindowOpenDisposition::NEW_BACKGROUND_TAB:
        disposition = "background-tab";
        break;
      case WindowOpenDisposition::NEW_POPUP:
      case WindowOpenDisposition::NEW_WINDOW:
        disposition = "new-window";
        break;
      case WindowOpenDisposition::SAVE_TO_DISK:
        disposition = "save-to-disk";
        break;
      default:
        break;
    }
    return mate::ConvertToV8(isolate, disposition);
  }
};
template<>
struct Converter<content::SavePageType> {
  static bool FromV8(v8::Isolate* isolate, v8::Local<v8::Value> val,