* 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
content::RenderFrameHost* frame_host) {
  bool handled = true;
  auto relay = NativeWindowRelay::FromWebContents(web_contents());
  if (!relay)
    return false;
  IPC_BEGIN_MESSAGE_MAP_WITH_PARAM(WebContents, message, frame_host)
    IPC_MESSAGE_HANDLER(AtomAutofillFrameHostMsg_ShowPopup, ShowAutofillPopup)
  IPC_END_MESSAGE_MAP()
  IPC_BEGIN_MESSAGE_MAP_WITH_PARAM(NativeWindow, message, frame_host)
    IPC_MESSAGE_FORWARD(AtomAutofillFrameHostMsg_HidePopup,
      relay->window.get(), NativeWindow::HideAutofillPopup)
    IPC_MESSAGE_UNHANDLED(handled = false)
  IPC_END_MESSAGE_MAP()
  return handled;
}
void WebContents::WebContentsDestroyed() {