* 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
const base::Callback<void(bool)>& callback) {
  auto context = GetServiceWorkerContext(web_contents());
  if (!context)
    return;
  struct WrappedCallback {
    base::Callback<void(bool)> callback_;
    explicit WrappedCallback(const base::Callback<void(bool)>& callback)
        : callback_(callback) {}
    void Run(content::ServiceWorkerCapability capability) {
      callback_.Run(capability !=
                    content::ServiceWorkerCapability::NO_SERVICE_WORKER);
      delete this;
    }
  };
  auto wrapped_callback = new WrappedCallback(callback);
  context->CheckHasServiceWorker(
      web_contents()->GetLastCommittedURL(), GURL::EmptyGURL(),
      base::Bind(&WrappedCallback::Run, base::Unretained(wrapped_callback)));
}
void WebContents::UnregisterServiceWorker(
    const base::Callback<void(bool)>& callback) {
  auto context = GetServiceWorkerContext(web_contents());
  if (!context)
    return;
  context->UnregisterServiceWorker(web_contents()->GetLastCommittedURL(),
                                   callback);
}
void WebContents::SetIgnoreMenuShortcuts(bool ignore) {