* 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
return static_cast<GURL>(render_frame->GetWebFrame()->GetDocument().Url())
      .SchemeIs("chrome-extension");
}
}  
AtomRendererClient::AtomRendererClient()
    : node_integration_initialized_(false),
      node_bindings_(NodeBindings::Create(NodeBindings::RENDERER)),
      atom_bindings_(new AtomBindings(uv_default_loop())) {
  isolated_world_ = base::CommandLine::ForCurrentProcess()->HasSwitch(
      switches::kContextIsolation);
}
AtomRendererClient::~AtomRendererClient() {
  asar::ClearArchives();
}
void AtomRendererClient::RenderThreadStarted() {