* 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::WebContents* source_contents,
    int opener_render_process_id,
    int opener_render_frame_id,
    const std::string& frame_name,
    const GURL& target_url,
    content::WebContents* new_contents) {
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  auto api_web_contents = CreateFrom(isolate(), new_contents, BROWSER_WINDOW);
  Emit("-web-contents-created", api_web_contents, target_url, frame_name);
}
void WebContents::AddNewContents(content::WebContents* source,
                                 content::WebContents* new_contents,
                                 WindowOpenDisposition disposition,
                                 const gfx::Rect& initial_rect,
                                 bool user_gesture,
                                 bool* was_blocked) {
  new ChildWebContentsTracker(new_contents);
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  auto api_web_contents = CreateFrom(isolate(), new_contents);
  if (Emit("-add-new-contents", api_web_contents, disposition, user_gesture,
      initial_rect.x(), initial_rect.y(), initial_rect.width(),
      initial_rect.height())) {
    api_web_contents->DestroyWebContents(true );
  }
}
content::WebContents* WebContents::OpenURLFromTab(