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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
base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
  mate::Dictionary dict(isolate, binding);
  if (command_line->HasSwitch(switches::kGuestInstanceID))
    dict.Set(options::kGuestInstanceID,
             command_line->GetSwitchValueASCII(switches::kGuestInstanceID));
  if (command_line->HasSwitch(switches::kOpenerID))
    dict.Set(options::kOpenerID,
             command_line->GetSwitchValueASCII(switches::kOpenerID));
  dict.Set("hiddenPage", command_line->HasSwitch(switches::kHiddenPage));
  dict.Set(options::kNativeWindowOpen,
           command_line->HasSwitch(switches::kNativeWindowOpen));
  v8::Local<v8::Value> args[] = {binding};
  ignore_result(
      result.As<v8::Function>()->Call(context, v8::Null(isolate), 1, args));
}
node::Environment* AtomRendererClient::GetEnvironment(
    content::RenderFrame* render_frame) const {
  if (injected_frames_.find(render_frame) == injected_frames_.end())
    return nullptr;
  v8::HandleScope handle_scope(v8::Isolate::GetCurrent());
  auto context =
      GetContext(render_frame->GetWebFrame(), v8::Isolate::GetCurrent());
  node::Environment* env = node::Environment::GetCurrent(context);
  if (environments_.find(env) == environments_.end())
    return nullptr;
  return env;
}
}