* 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.cc
const GURL& url,
                                    const std::string& http_method,
                                    bool is_initial_navigation,
                                    bool is_server_redirect,
                                    bool* send_referrer) {
  
  
  
  
  *send_referrer = true;
  return http_method == "GET";
}
void AtomRendererClient::DidInitializeWorkerContextOnWorkerThread(