**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
content::RenderFrame* main_frame)
      : spell_check_client_(spell_check_client), main_frame_(main_frame) {
    content::RenderFrame::ForEach(this);
    main_frame->GetWebFrame()->SetSpellCheckPanelHostClient(spell_check_client);
  }
  bool Visit(content::RenderFrame* render_frame) override {
    auto* view = render_frame->GetRenderView();
    if (view->GetMainRenderFrame() == main_frame_ ||
        (render_frame->IsMainFrame() && render_frame == main_frame_)) {
      render_frame->GetWebFrame()->SetTextCheckClient(spell_check_client_);
    }
    return true;
  }
 private:
  SpellCheckClient* spell_check_client_;
  content::RenderFrame* main_frame_;
  DISALLOW_COPY_AND_ASSIGN(FrameSetSpellChecker);
};
class SpellCheckerHolder : public content::RenderFrameObserver {
 public:
  
  static SpellCheckerHolder* FromRenderFrame(
      content::RenderFrame* render_frame) {
    for (auto* holder : instances_) {
      if (holder->render_frame() == render_frame)
        return holder;
    }
    return nullptr;
  }
  SpellCheckerHolder(content::RenderFrame* render_frame,