* 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
return window_->GetOpacity();
}
void Window::FocusOnWebView() {
  window_->FocusOnWebView();
}
void Window::BlurWebView() {
  window_->BlurWebView();
}
bool Window::IsWebViewFocused() {
  return window_->IsWebViewFocused();
}
void Window::SetRepresentedFilename(const std::string& filename) {
  window_->SetRepresentedFilename(filename);
}
std::string Window::GetRepresentedFilename() {
  return window_->GetRepresentedFilename();
}
void Window::SetDocumentEdited(bool edited) {
  window_->SetDocumentEdited(edited);
}
bool Window::IsDocumentEdited() {
  return window_->IsDocumentEdited();
}
void Window::SetIgnoreMouseEvents(bool ignore, mate::Arguments* args) {
  mate::Dictionary options;
  bool forward = false;
  args->GetNext(&options) && options.Get("forward", &forward);
  return window_->SetIgnoreMouseEvents(ignore, forward);
}
void Window::SetContentProtection(bool enable) {