* 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
bool silent;
  bool print_background;
  base::string16 device_name;
};
}  
namespace mate {
template<>
struct Converter<atom::SetSizeParams> {
  static bool FromV8(v8::Isolate* isolate,
                     v8::Local<v8::Value> val,
                     atom::SetSizeParams* out) {
    mate::Dictionary params;
    if (!ConvertFromV8(isolate, val, &params))
      return false;
    bool autosize;
    if (params.Get("enableAutoSize", &autosize))
      out->enable_auto_size.reset(new bool(true));
    gfx::Size size;
    if (params.Get("min", &size))
      out->min_size.reset(new gfx::Size(size));
    if (params.Get("max", &size))
      out->max_size.reset(new gfx::Size(size));
    if (params.Get("normal", &size))
      out->normal_size.reset(new gfx::Size(size));
    return true;
  }
};
template<>
struct Converter<PrintSettings> {
  static bool FromV8(v8::Isolate* isolate, v8::Local<v8::Value> val,