* 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/renderer_client_base.h
#ifndef ATOM_RENDERER_RENDERER_CLIENT_BASE_H_
#define ATOM_RENDERER_RENDERER_CLIENT_BASE_H_
#include <string>
#include <vector>
#include "content/public/renderer/content_renderer_client.h"
namespace atom {
class PreferencesManager;
class RendererClientBase : public content::ContentRendererClient {
 public:
  RendererClientBase();
  virtual ~RendererClientBase();
  virtual void DidCreateScriptContext(
      v8::Handle<v8::Context> context, content::RenderFrame* render_frame) = 0;
  virtual void WillReleaseScriptContext(