<!--  As an open source project with a dedicated but small maintainer team, it can sometimes take a long time for issues to be addressed so please be patient and we will get back to you as soon as we can.
-->

### Preflight Checklist
<!-- Please ensure you've completed the following steps by replacing [ ] with [x]-->

* [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
public:
  explicit ScriptExecutionCallback(electron::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:
  electron::util::Promise promise_;
  DISALLOW_COPY_AND_ASSIGN(ScriptExecutionCallback);
};
class FrameSetSpellChecker : public content::RenderFrameVisitor {
 public:
  FrameSetSpellChecker(SpellCheckClient* spell_check_client,