* 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
#ifndef ATOM_RENDERER_ATOM_SANDBOXED_RENDERER_CLIENT_H_
#define ATOM_RENDERER_ATOM_SANDBOXED_RENDERER_CLIENT_H_
#include <string>
#include <vector>
#include "atom/renderer/renderer_client_base.h"
namespace atom {
class AtomSandboxedRendererClient : public RendererClientBase {