* 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
v8::Local<v8::Context> context) {
  if (base::CommandLine::ForCurrentProcess()->HasSwitch(
          switches::kNodeIntegrationInWorker)) {
    WebWorkerObserver::GetCurrent()->ContextCreated(context);
  }
}
void AtomRendererClient::WillDestroyWorkerContextOnWorkerThread(
    v8::Local<v8::Context> context) {
  if (base::CommandLine::ForCurrentProcess()->HasSwitch(
          switches::kNodeIntegrationInWorker)) {
    WebWorkerObserver::GetCurrent()->ContextWillDestroy(context);
  }
}
v8::Local<v8::Context> AtomRendererClient::GetContext(
    blink::WebFrame* frame, v8::Isolate* isolate) {
  if (isolated_world())
    return frame->WorldScriptContext(isolate, World::ISOLATED_WORLD);
  else
    return frame->MainWorldScriptContext();
}
void AtomRendererClient::SetupMainWorldOverrides(
    v8::Handle<v8::Context> context) {