**Is your feature request related to a problem? Please describe.**
After webContents.insertCSS() I cant find a way to remove injected stylesheet.

**Describe the solution you'd like**
insert: webContents.insertCSS('style-id', 'body { ... }') or .inserCSS('body { ... })'
remove: webContents.removeCSS('style-id') or .removeCSS() to remove all injected stylesheets


atom/renderer/api/atom_api_web_frame.cc
v8::Local<v8::Value> window,
                                     v8::Local<v8::Object> context,
                                     v8::Local<v8::Function> register_cb) {
  v8::HandleScope handle_scope(isolate);
  v8::Context::Scope context_scope(context->CreationContext());
  blink::WebCustomElement::EmbedderNamesAllowedScope embedder_names_scope;
  GetRenderFrame(context)->GetWebFrame()->RequestExecuteV8Function(
      context->CreationContext(), register_cb, v8::Null(isolate), 0, nullptr,
      nullptr);
}
int GetWebFrameId(v8::Local<v8::Value> window,
                  v8::Local<v8::Value> content_window) {
  
  
  
  content::RenderFrame* render_frame = GetRenderFrame(content_window);
  RenderFrameStatus render_frame_status(render_frame);
  if (!render_frame_status.is_ok())
    return -1;
  blink::WebLocalFrame* frame = render_frame->GetWebFrame();
  
  blink::WebFrame* parent_frame = frame->Parent();
  DCHECK(parent_frame);
  DCHECK(parent_frame->IsWebLocalFrame());
  return render_frame->GetRoutingID();
}
void SetSpellCheckProvider(mate::Arguments* args,