* 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
content::RenderFrame* render_frame) {
  new AtomRenderFrameObserver(render_frame, this);
  new AutofillAgent(render_frame);
  new PepperHelper(render_frame);
  new ContentSettingsObserver(render_frame);
  new printing::PrintWebViewHelper(render_frame);
  
  
  blink::WebSecurityPolicy::RegisterURLSchemeAsAllowingServiceWorkers("file");
  
  blink::ResetPluginCache();
  
  blink::WebSecurityPolicy::AddOriginAccessWhitelistEntry(
      GURL(kPdfViewerUIOrigin), "file", "", true);
}
void RendererClientBase::RenderViewCreated(content::RenderView* render_view) {