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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
return world_id == World::MAIN_WORLD;
}
bool AtomRenderFrameObserver::IsIsolatedWorld(int world_id) {
  return world_id == World::ISOLATED_WORLD;
}
bool AtomRenderFrameObserver::ShouldNotifyClient(int world_id) {
  if (renderer_client_->isolated_world() && render_frame_->IsMainFrame())
    return IsIsolatedWorld(world_id);
  else
    return IsMainWorld(world_id);
}
bool AtomRenderFrameObserver::OnMessageReceived(const IPC::Message& message) {
  bool handled = true;
  IPC_BEGIN_MESSAGE_MAP(AtomRenderFrameObserver, message)
    IPC_MESSAGE_HANDLER(AtomFrameMsg_Message, OnBrowserMessage)
    IPC_MESSAGE_HANDLER(AtomFrameMsg_TakeHeapSnapshot, OnTakeHeapSnapshot)
    IPC_MESSAGE_UNHANDLED(handled = false)
  IPC_END_MESSAGE_MAP()
  return handled;
}
void AtomRenderFrameObserver::OnBrowserMessage(bool internal,