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* Electron version: 1.7.2
* Operating system: macOS Sierra

### Expected behavior

Webview appear

### Actual behavior

It don't

### How to reproduce

1. Create a browser window with contextIsolation on true and webviewTag on true
2. Inside of it, add a webview
3. Run it, webview should not appear

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atom/renderer/atom_renderer_client.cc
v8::Local<v8::Context> context) {
  if (base::CommandLine::ForCurrentProcess()->HasSwitch(
          switches::kNodeIntegrationInWorker)) {
    WebWorkerObserver::GetCurrent()->ContextWillDestroy(context);
  }
}
void AtomRendererClient::SetupMainWorldOverrides(
    v8::Handle<v8::Context> context) {
  
  v8::Isolate* isolate = context->GetIsolate();
  v8::HandleScope handle_scope(isolate);
  v8::Context::Scope context_scope(context);
  
  
  std::string left = "(function (binding, require) {\n";
  std::string right = "\n})";
  auto source = v8::String::Concat(
      isolate, mate::ConvertToV8(isolate, left)->ToString(isolate),
      v8::String::Concat(isolate,
                         node::isolated_bundle_value.ToStringChecked(isolate),
                         mate::ConvertToV8(isolate, right)->ToString(isolate)));
  auto result = RunScript(context, source);
  DCHECK(result->IsFunction());
  auto binding = v8::Object::New(isolate);
  api::Initialize(binding, v8::Null(isolate), context, nullptr);