* Electron version: 1.7.3
* Operating system: Windows, MacOS, Linux

### Expected behavior
Context Isolation should work when sandbox option is set to true

### Actual behavior
Context Isolation should work when sandbox option is set to true

### How to reproduce

Turn on contextIsolation and sandbox on browser window. contextIsolation doesn't work. 

contextIsolation work needs to happen on sandbox renderer client as well. 

cc: @tarruda @kevinsawicki @chandrachivukula
atom/renderer/atom_renderer_client.h
EXCEPT_IFRAME,
    MANUAL_ENABLE_IFRAME,
    DISABLE,
  };
  
  void RenderThreadStarted() override;
  void RenderFrameCreated(content::RenderFrame*) override;
  void RenderViewCreated(content::RenderView*) override;
  void RunScriptsAtDocumentStart(content::RenderFrame* render_frame) override;
  void RunScriptsAtDocumentEnd(content::RenderFrame* render_frame) override;
  bool ShouldFork(blink::WebLocalFrame* frame,
                  const GURL& url,
                  const std::string& http_method,
                  bool is_initial_navigation,
                  bool is_server_redirect,
                  bool* send_referrer) override;
  void DidInitializeWorkerContextOnWorkerThread(
      v8::Local<v8::Context> context) override;
  void WillDestroyWorkerContextOnWorkerThread(
      v8::Local<v8::Context> context) override;
  
  bool node_integration_initialized_;
  std::unique_ptr<NodeBindings> node_bindings_;
  std::unique_ptr<AtomBindings> atom_bindings_;
  bool isolated_world_;
  DISALLOW_COPY_AND_ASSIGN(AtomRendererClient);
};
}  
#endif