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### Preflight Checklist
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* [x] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
* [x] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
* [x] I have searched the issue tracker for a feature request that matches the one I want to file, without success.

### Problem Description
<!-- Is your feature request related to a problem? Please add a clear and concise description of what the problem is. -->

Hi, I'm trying to integrate uBlock Origin extension into my app, using my own package [electron-extensions](https://github.com/sentialx/electron-extensions). But I can see uBlock Origin uses `chrome.tabs.insertCSS` with `cssOrigin` set to `user`. The property is important to block elements with `display: block !important`.

### Proposed Solution
<!-- Describe the solution you'd like in a clear and concise manner -->
In `webContents.insertCSS` there should be a `details` object with at least `cssOrigin` property as it's described [here](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/insertCSS).

### Alternatives Considered
Unfortunately there are no alternatives. If there were any, I wouldn't create this issue.

shell/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,