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* Electron version: 1.7.2
* Operating system: macOS Sierra

### Expected behavior

Webview appear

### Actual behavior

It don't

### How to reproduce

1. Create a browser window with contextIsolation on true and webviewTag on true
2. Inside of it, add a webview
3. Run it, webview should not appear

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atom/renderer/atom_render_frame_observer.cc
v8::Isolate* isolate,
    const base::ListValue& list) {
  v8::Local<v8::Value> array = mate::ConvertToV8(isolate, list);
  std::vector<v8::Local<v8::Value>> result;
  mate::ConvertFromV8(isolate, array, &result);
  return result;
}
base::StringPiece NetResourceProvider(int key) {
  if (key == IDR_DIR_HEADER_HTML) {
    base::StringPiece html_data =
        ui::ResourceBundle::GetSharedInstance().GetRawDataResource(
            IDR_DIR_HEADER_HTML);
    return html_data;
  }
  return base::StringPiece();
}
}  
AtomRenderFrameObserver::AtomRenderFrameObserver(
    content::RenderFrame* frame,
    RendererClientBase* renderer_client)
    : content::RenderFrameObserver(frame),
      render_frame_(frame),
      renderer_client_(renderer_client) {
  
  net::NetModule::SetResourceProvider(NetResourceProvider);
}
void AtomRenderFrameObserver::DidClearWindowObject() {
  renderer_client_->DidClearWindowObject(render_frame_);
}
void AtomRenderFrameObserver::DidCreateDocumentElement() {