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* Electron version: 2.0.0-beta.7
* Operating system: Windows 10 x64

### Expected behavior

<!-- What do you think should happen? -->
When `webPreferences.nativeWindowOpen` is set to true, BrowserWindows opened by `window.open`'s memory will be released after be closed.

### Actual behavior

<!-- What actually happens? -->
The memory is not released and caused memory leak

### How to reproduce
- Clone https://github.com/KochiyaOcean/electron-bugs/tree/native-open-memory-leak
- Run `electron . ` in the cloned repo
- Memory usage of electron process will be increasing in task manager

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atom/browser/web_contents_preferences.cc
#include "atom/browser/web_contents_preferences.h"
#include <algorithm>
#include <string>
#include <utility>
#include <vector>
#include "atom/browser/native_window.h"
#include "atom/browser/web_view_manager.h"
#include "atom/common/native_mate_converters/value_converter.h"
#include "atom/common/options_switches.h"
#include "base/command_line.h"
#include "base/memory/ptr_util.h"
#include "base/strings/string_number_conversions.h"
#include "base/strings/utf_string_conversions.h"
#include "cc/base/switches.h"
#include "content/public/browser/render_frame_host.h"
#include "content/public/browser/render_process_host.h"
#include "content/public/common/content_switches.h"
#include "content/public/common/web_preferences.h"
#include "native_mate/dictionary.h"
#include "net/base/filename_util.h"
#include "services/service_manager/sandbox/switches.h"
#if defined(OS_WIN)
#include "ui/gfx/switches.h"
#endif
DEFINE_WEB_CONTENTS_USER_DATA_KEY(atom::WebContentsPreferences);
namespace {
bool GetAsString(const base::Value* val,