* 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.cc
blink::WebFrameWidget* web_frame_widget = render_view->GetWebFrameWidget();
  if (!web_frame_widget)
    return;
  base::CommandLine* cmd = base::CommandLine::ForCurrentProcess();
  if (cmd->HasSwitch(switches::kGuestInstanceID)) {  
    web_frame_widget->SetBaseBackgroundColor(SK_ColorTRANSPARENT);
  } else {  
    
    std::string name = cmd->GetSwitchValueASCII(switches::kBackgroundColor);
    
    SkColor color = name.empty() ? SK_ColorWHITE : ParseHexColor(name);
    web_frame_widget->SetBaseBackgroundColor(color);
  }
}
void RendererClientBase::DidClearWindowObject(
    content::RenderFrame* render_frame) {
  
  render_frame->GetWebFrame()->ExecuteScript(blink::WebScriptSource("void 0"));
}
blink::WebSpeechSynthesizer* RendererClientBase::OverrideSpeechSynthesizer(
    blink::WebSpeechSynthesizerClient* client) {
  return new TtsDispatcher(client);
}
bool RendererClientBase::OverrideCreatePlugin(