* 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,
    blink::WebLocalFrame* frame,
    const blink::WebPluginParams& params,
    blink::WebPlugin** plugin) {
  base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
  if (params.mime_type.Utf8() == content::kBrowserPluginMimeType ||
      params.mime_type.Utf8() == kPdfPluginMimeType ||
      command_line->HasSwitch(switches::kEnablePlugins))
    return false;
  *plugin = nullptr;
  return true;
}
content::BrowserPluginDelegate* RendererClientBase::CreateBrowserPluginDelegate(
    content::RenderFrame* render_frame,
    const std::string& mime_type,
    const GURL& original_url) {
  if (mime_type == content::kBrowserPluginMimeType) {
    return new GuestViewContainer(render_frame);
  } else {
    return nullptr;
  }
}
void RendererClientBase::AddSupportedKeySystems(
    std::vector<std::unique_ptr<::media::KeySystemProperties>>* key_systems) {
  AddChromeKeySystems(key_systems);
}
}