**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,
                           const std::string& language,
                           v8::Local<v8::Object> provider) {
  auto context = args->isolate()->GetCurrentContext();
  if (!provider->Has(context, mate::StringToV8(args->isolate(), "spellCheck"))
           .ToChecked()) {
    args->ThrowError("\"spellCheck\" has to be defined");
    return;
  }
  
  content::RenderFrame* render_frame = GetRenderFrame(window);
  auto* existing = SpellCheckerHolder::FromRenderFrame(render_frame);
  if (existing)
    existing->UnsetAndDestroy();
  
  
  auto spell_check_client =
      std::make_unique<SpellCheckClient>(language, args->isolate(), provider);
  FrameSetSpellChecker spell_checker(spell_check_client.get(), render_frame);
  
  new SpellCheckerHolder(render_frame, std::move(spell_check_client));
}
void InsertText(v8::Local<v8::Value> window, const std::string& text) {