**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()->GetDocument().InsertStyleSheet(
        blink::WebString::FromUTF8(css));
  }
}
v8::Local<v8::Promise> ExecuteJavaScript(mate::Arguments* args,
                                         v8::Local<v8::Value> window,
                                         const base::string16& code) {
  v8::Isolate* isolate = args->isolate();
  util::Promise promise(isolate);
  v8::Local<v8::Promise> handle = promise.GetHandle();
  bool has_user_gesture = false;
  args->GetNext(&has_user_gesture);
  GetRenderFrame(window)->GetWebFrame()->RequestExecuteScriptAndReturnValue(
      blink::WebScriptSource(blink::WebString::FromUTF16(code)),
      has_user_gesture, new ScriptExecutionCallback(std::move(promise)));
  return handle;
}
v8::Local<v8::Promise> ExecuteJavaScriptInIsolatedWorld(