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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
return static_cast<GURL>(render_frame->GetWebFrame()->GetDocument().Url())
      .SchemeIs("chrome-extension");
}
}  
AtomRendererClient::AtomRendererClient()
    : node_bindings_(NodeBindings::Create(NodeBindings::RENDERER)),
      atom_bindings_(new AtomBindings(uv_default_loop())) {}
AtomRendererClient::~AtomRendererClient() {
  asar::ClearArchives();
}
void AtomRendererClient::RenderThreadStarted() {
  RendererClientBase::RenderThreadStarted();
}
void AtomRendererClient::RenderFrameCreated(
    content::RenderFrame* render_frame) {
  new AtomRenderFrameObserver(render_frame, this);
  RendererClientBase::RenderFrameCreated(render_frame);
}
void AtomRendererClient::RenderViewCreated(content::RenderView* render_view) {
  RendererClientBase::RenderViewCreated(render_view);
}
void AtomRendererClient::RunScriptsAtDocumentStart(