**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
double result = 0.0;
  content::RenderFrame* render_frame = GetRenderFrame(window);
  mojom::ElectronBrowserPtr browser_ptr;
  render_frame->GetRemoteInterfaces()->GetInterface(
      mojo::MakeRequest(&browser_ptr));
  browser_ptr->DoGetZoomLevel(&result);
  return result;
}
void SetZoomFactor(v8::Local<v8::Value> window, double factor) {
  SetZoomLevel(window, blink::WebView::ZoomFactorToZoomLevel(factor));
}
double GetZoomFactor(v8::Local<v8::Value> window) {
  return blink::WebView::ZoomLevelToZoomFactor(GetZoomLevel(window));
}
void SetVisualZoomLevelLimits(v8::Local<v8::Value> window,
                              double min_level,
                              double max_level) {
  blink::WebFrame* web_frame = GetRenderFrame(window)->GetWebFrame();
  web_frame->View()->SetDefaultPageScaleLimits(min_level, max_level);
  web_frame->View()->SetIgnoreViewportTagScaleLimits(true);
}
void SetLayoutZoomLevelLimits(v8::Local<v8::Value> window,
                              double min_level,
                              double max_level) {
  blink::WebFrame* web_frame = GetRenderFrame(window)->GetWebFrame();
  web_frame->View()->ZoomLimitsChanged(min_level, max_level);
}
void AllowGuestViewElementDefinition(v8::Isolate* isolate,