* 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
.SetMethod("selectAll", &WebContents::SelectAll)
      .SetMethod("unselect", &WebContents::Unselect)
      .SetMethod("replace", &WebContents::Replace)
      .SetMethod("replaceMisspelling", &WebContents::ReplaceMisspelling)
      .SetMethod("findInPage", &WebContents::FindInPage)
      .SetMethod("stopFindInPage", &WebContents::StopFindInPage)
      .SetMethod("focus", &WebContents::Focus)
      .SetMethod("isFocused", &WebContents::IsFocused)
      .SetMethod("tabTraverse", &WebContents::TabTraverse)
      .SetMethod("_send", &WebContents::SendIPCMessage)
      .SetMethod("sendInputEvent", &WebContents::SendInputEvent)
      .SetMethod("beginFrameSubscription", &WebContents::BeginFrameSubscription)
      .SetMethod("endFrameSubscription", &WebContents::EndFrameSubscription)
      .SetMethod("startDrag", &WebContents::StartDrag)
      .SetMethod("setSize", &WebContents::SetSize)
      .SetMethod("isGuest", &WebContents::IsGuest)
#if defined(ENABLE_OSR)
      .SetMethod("isOffscreen", &WebContents::IsOffScreen)
#endif
      .SetMethod("startPainting", &WebContents::StartPainting)
      .SetMethod("stopPainting", &WebContents::StopPainting)
      .SetMethod("isPainting", &WebContents::IsPainting)
      .SetMethod("setFrameRate", &WebContents::SetFrameRate)
      .SetMethod("getFrameRate", &WebContents::GetFrameRate)
      .SetMethod("invalidate", &WebContents::Invalidate)
      .SetMethod("setZoomLevel", &WebContents::SetZoomLevel)
      .SetMethod("_getZoomLevel", &WebContents::GetZoomLevel)
      .SetMethod("setZoomFactor", &WebContents::SetZoomFactor)
      .SetMethod("_getZoomFactor", &WebContents::GetZoomFactor)
      .SetMethod("getType", &WebContents::GetType)