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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_sandboxed_renderer_client.h
public:
  AtomSandboxedRendererClient();
  ~AtomSandboxedRendererClient() override;
  void InitializeBindings(v8::Local<v8::Object> binding,
                          v8::Local<v8::Context> context);
  void InvokeIpcCallback(v8::Handle<v8::Context> context,
                         const std::string& callback_name,
                         std::vector<v8::Handle<v8::Value>> args);
  
  void DidCreateScriptContext(v8::Handle<v8::Context> context,
                              content::RenderFrame* render_frame) override;
  void WillReleaseScriptContext(v8::Handle<v8::Context> context,
                                content::RenderFrame* render_frame) override;
  void SetupMainWorldOverrides(v8::Handle<v8::Context> context) override {}
  
  void RenderFrameCreated(content::RenderFrame*) override;
  void RenderViewCreated(content::RenderView*) override;
 private:
  std::unique_ptr<base::ProcessMetrics> metrics_;
  DISALLOW_COPY_AND_ASSIGN(AtomSandboxedRendererClient);
};
}  
#endif