* 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::WebCustomElement::AddEmbedderCustomElementName("webview");
  blink::WebCustomElement::AddEmbedderCustomElementName("browserplugin");
  
  std::vector<std::string> secure_schemes_list =
      ParseSchemesCLISwitch(switches::kSecureSchemes);
  for (const std::string& scheme : secure_schemes_list)
    blink::SchemeRegistry::RegisterURLSchemeAsSecure(
        WTF::String::FromUTF8(scheme.data(), scheme.length()));
  preferences_manager_.reset(new PreferencesManager);
#if defined(OS_WIN)
  
  base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
  base::string16 app_id =
      command_line->GetSwitchValueNative(switches::kAppUserModelId);
  if (!app_id.empty()) {
    SetCurrentProcessExplicitAppUserModelID(app_id.c_str());
  }
#endif
#if defined(OS_MACOSX)
  base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
  bool scroll_bounce = command_line->HasSwitch(switches::kScrollBounce);
  base::ScopedCFTypeRef<CFStringRef> rubber_banding_key(
    base::SysUTF8ToCFStringRef("NSScrollViewRubberbanding"));
  CFPreferencesSetAppValue(rubber_banding_key,
                           scroll_bounce ? kCFBooleanTrue : kCFBooleanFalse,
                           kCFPreferencesCurrentApplication);
  CFPreferencesAppSynchronize(kCFPreferencesCurrentApplication);
#endif
}
void RendererClientBase::RenderFrameCreated(