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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
std::string* value) const {
  return GetAsString(&preference_, name, value);
}
bool WebContentsPreferences::IsRemoteModuleEnabled() const {
  return IsEnabled(options::kEnableRemoteModule, true);
}
bool WebContentsPreferences::GetPreloadPath(
    base::FilePath::StringType* path) const {
  DCHECK(path);
  base::FilePath::StringType preload;
  if (GetAsString(&preference_, options::kPreloadScript, &preload)) {
    if (base::FilePath(preload).IsAbsolute()) {
      *path = std::move(preload);
      return true;
    } else {
      LOG(ERROR) << "preload script must have absolute path.";
    }
  } else if (GetAsString(&preference_, options::kPreloadURL, &preload)) {
    
    base::FilePath preload_path;
    if (net::FileURLToFilePath(GURL(preload), &preload_path)) {
      *path = std::move(preload_path.value());
      return true;
    } else {
      LOG(ERROR) << "preload url must be file:// protocol.";
    }
  }
  return false;
}
content::WebContents* WebContentsPreferences::GetWebContentsFromProcessID(