* 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::Handle<v8::Context> context, content::RenderFrame* render_frame) {
  
  
  if (!render_frame->IsMainFrame() && !IsDevToolsExtension(render_frame))
    return;
  node::Environment* env = node::Environment::GetCurrent(context);
  if (env)
    mate::EmitEvent(env->isolate(), env->process_object(), "exit");
  
  if (env == node_bindings_->uv_env())
    node_bindings_->set_uv_env(nullptr);
  
  
  
  
  
  
  atom_bindings_->EnvironmentDestroyed(env);
}
bool AtomRendererClient::ShouldFork(blink::WebLocalFrame* frame,