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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/renderer/atom_renderer_client.cc
v8::Local<v8::Context> context) {
  if (base::CommandLine::ForCurrentProcess()->HasSwitch(
          switches::kNodeIntegrationInWorker)) {
    WebWorkerObserver::GetCurrent()->ContextCreated(context);
  }
}
void AtomRendererClient::WillDestroyWorkerContextOnWorkerThread(
    v8::Local<v8::Context> context) {
  if (base::CommandLine::ForCurrentProcess()->HasSwitch(
          switches::kNodeIntegrationInWorker)) {
    WebWorkerObserver::GetCurrent()->ContextWillDestroy(context);
  }
}
void AtomRendererClient::SetupMainWorldOverrides(
    v8::Handle<v8::Context> context) {
  
  v8::Isolate* isolate = context->GetIsolate();
  
  
  std::string left = "(function (binding, require) {\n";
  std::string right = "\n})";
  auto script = v8::Script::Compile(v8::String::Concat(
      mate::ConvertToV8(isolate, left)->ToString(),
      v8::String::Concat(node::isolated_bundle_value.ToStringChecked(isolate),
                         mate::ConvertToV8(isolate, right)->ToString())));
  auto func =
      v8::Handle<v8::Function>::Cast(script->Run(context).ToLocalChecked());
  auto binding = v8::Object::New(isolate);