**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
blink::WebFrame* web_frame = GetRenderFrame(window)->GetWebFrame();
  if (web_frame->IsWebLocalFrame()) {
    web_frame->ToWebLocalFrame()
        ->FrameWidget()
        ->GetActiveWebInputMethodController()
        ->CommitText(blink::WebString::FromUTF8(text),
                     blink::WebVector<blink::WebImeTextSpan>(),
                     blink::WebRange(), 0);
  }
}
void InsertCSS(v8::Local<v8::Value> window, const std::string& css) {