* 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
AtomViewHostMsg_GetZoomLevel::WriteReplyParams(reply_msg, GetZoomLevel());
  Send(reply_msg);
}
v8::Local<v8::Value> WebContents::GetWebPreferences(v8::Isolate* isolate) {
  WebContentsPreferences* web_preferences =
      WebContentsPreferences::FromWebContents(web_contents());
  return mate::ConvertToV8(isolate, *web_preferences->web_preferences());
}
v8::Local<v8::Value> WebContents::GetOwnerBrowserWindow() {
  if (owner_window())
    return Window::From(isolate(), owner_window());
  else
    return v8::Null(isolate());
}
int32_t WebContents::ID() const {
  return weak_map_id();
}
v8::Local<v8::Value> WebContents::Session(v8::Isolate* isolate) {
  return v8::Local<v8::Value>::New(isolate, session_);
}
content::WebContents* WebContents::HostWebContents() {
  if (!embedder_)
    return nullptr;
  return embedder_->web_contents();
}
void WebContents::SetEmbedder(const WebContents* embedder) {