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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
const base::StringPiece& path,
                 std::string* out) {
  if (val) {
    auto* found = val->FindKeyOfType(path, base::Value::Type::STRING);
    if (found) {
      *out = found->GetString();
      return true;
    }
  }
  return false;
}
bool GetAsString(const base::Value* val,