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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
mate::Arguments* args,
    v8::Local<v8::Value> window,
    int world_id,
    const std::vector<mate::Dictionary>& scripts) {
  v8::Isolate* isolate = args->isolate();
  util::Promise promise(isolate);
  v8::Local<v8::Promise> handle = promise.GetHandle();
  std::vector<blink::WebScriptSource> sources;
  for (const auto& script : scripts) {
    base::string16 code;
    base::string16 url;
    int start_line = 1;
    script.Get("url", &url);
    script.Get("startLine", &start_line);
    if (!script.Get("code", &code)) {
      promise.RejectWithErrorMessage("Invalid 'code'");
      return handle;
    }
    sources.emplace_back(
        blink::WebScriptSource(blink::WebString::FromUTF16(code),
                               blink::WebURL(GURL(url)), start_line));
  }
  bool has_user_gesture = false;
  args->GetNext(&has_user_gesture);
  blink::WebLocalFrame::ScriptExecutionType scriptExecutionType =
      blink::WebLocalFrame::kSynchronous;