**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
public:
  explicit ScriptExecutionCallback(atom::util::Promise promise)
      : promise_(std::move(promise)) {}
  ~ScriptExecutionCallback() override {}
  void Completed(
      const blink::WebVector<v8::Local<v8::Value>>& result) override {
    if (!result.empty()) {
      if (!result[0].IsEmpty()) {
        
        promise_.Resolve(result[0]);
      } else {
        promise_.RejectWithErrorMessage(
            "Script failed to execute, this normally means an error "
            "was thrown. Check the renderer console for the error.");
      }
    } else {
      promise_.RejectWithErrorMessage(
          "WebFrame was removed before script could run. This normally means "
          "the underlying frame was destroyed");
    }
    delete this;
  }
 private:
  atom::util::Promise promise_;
  DISALLOW_COPY_AND_ASSIGN(ScriptExecutionCallback);
};
class FrameSetSpellChecker : public content::RenderFrameVisitor {
 public:
  FrameSetSpellChecker(SpellCheckClient* spell_check_client,