* 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_sandboxed_renderer_client.h
public:
  AtomSandboxedRendererClient();
  virtual ~AtomSandboxedRendererClient();
  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 { }
  bool isolated_world() override { return false; }
  
  void RenderFrameCreated(content::RenderFrame*) override;
  void RenderViewCreated(content::RenderView*) override;
 private:
  DISALLOW_COPY_AND_ASSIGN(AtomSandboxedRendererClient);
};
}  
#endif