* 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
std::vector<printing::PrinterBasicInfo> printers;
  auto print_backend = printing::PrintBackend::CreateInstance(nullptr);
  base::ThreadRestrictions::ScopedAllowIO allow_io;
  print_backend->EnumeratePrinters(&printers);
  return printers;
}
void WebContents::PrintToPDF(const base::DictionaryValue& setting,
                             const PrintToPDFCallback& callback) {
  printing::PrintPreviewMessageHandler::FromWebContents(web_contents())->
      PrintToPDF(setting, callback);
}
void WebContents::AddWorkSpace(mate::Arguments* args,
                               const base::FilePath& path) {
  if (path.empty()) {
    args->ThrowError("path cannot be empty");
    return;
  }
  DevToolsAddFileSystem(path);
}
void WebContents::RemoveWorkSpace(mate::Arguments* args,