* Output of `node_modules/.bin/electron --version`: 4.0.0-beta.8
* Operating System (Platform and Version): Windows 10 1803
* Output of `node_modules/.bin/electron --version` on last known working Electron version (if applicable): 4.0.0-beta.7

**Expected Behavior**
`BrowserWindow.getNativeWindowHandle()` should return different value for different window

**Actual behavior**
`BrowserWindow.getNativeWindowHandle()` returns same value for different window, which is not true nativeWindowHandle for any of them.

**To Reproduce**
Open two `BrowserWindow`, and run `getNativeWindowHandle` on both `BrowserWindow`
```js
const { remote } = require('electron')
w1 = new remote.BrowserWindow()
w2 = new remote.BrowserWindow()
console.log(w1.getNativeWindowHandle(), w2.getNativeWindowHandle())
```
![image](https://user-images.githubusercontent.com/8194131/49328509-7411a800-f5b5-11e8-9a24-827aa589d937.png)


atom/browser/native_window.h
virtual void Hide() = 0;
  virtual bool IsVisible() = 0;
  virtual bool IsEnabled() = 0;
  virtual void SetEnabled(bool enable) = 0;
  virtual void Maximize() = 0;
  virtual void Unmaximize() = 0;
  virtual bool IsMaximized() = 0;
  virtual void Minimize() = 0;
  virtual void Restore() = 0;
  virtual bool IsMinimized() = 0;
  virtual void SetFullScreen(bool fullscreen) = 0;
  virtual bool IsFullscreen() const = 0;
  virtual void SetBounds(const gfx::Rect& bounds, bool animate = false) = 0;
  virtual gfx::Rect GetBounds() = 0;
  virtual void SetSize(const gfx::Size& size, bool animate = false);
  virtual gfx::Size GetSize();
  virtual void SetPosition(const gfx::Point& position, bool animate = false);
  virtual gfx::Point GetPosition();
  virtual void SetContentSize(const gfx::Size& size, bool animate = false);
  virtual gfx::Size GetContentSize();
  virtual void SetContentBounds(const gfx::Rect& bounds, bool animate = false);
  virtual gfx::Rect GetContentBounds();
  virtual bool IsNormal();
  virtual gfx::Rect GetNormalBounds() = 0;
  virtual void SetSizeConstraints(
      const extensions::SizeConstraints& size_constraints);
  virtual extensions::SizeConstraints GetSizeConstraints() const;
  virtual void SetContentSizeConstraints(
      const extensions::SizeConstraints& size_constraints);
  virtual extensions::SizeConstraints GetContentSizeConstraints() const;