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### 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
dict.SetMethod("setZoomFactor", &SetZoomFactor);
  dict.SetMethod("getZoomFactor", &GetZoomFactor);
  dict.SetMethod("setVisualZoomLevelLimits", &SetVisualZoomLevelLimits);
  dict.SetMethod("setLayoutZoomLevelLimits", &SetLayoutZoomLevelLimits);
  dict.SetMethod("allowGuestViewElementDefinition",
                 &AllowGuestViewElementDefinition);
  dict.SetMethod("getWebFrameId", &GetWebFrameId);
  dict.SetMethod("setSpellCheckProvider", &SetSpellCheckProvider);
  dict.SetMethod("insertText", &InsertText);
  dict.SetMethod("insertCSS", &InsertCSS);
  dict.SetMethod("removeInsertedCSS", &RemoveInsertedCSS);
  dict.SetMethod("executeJavaScript", &ExecuteJavaScript);
  dict.SetMethod("executeJavaScriptInIsolatedWorld",
                 &ExecuteJavaScriptInIsolatedWorld);
  dict.SetMethod("setIsolatedWorldInfo", &SetIsolatedWorldInfo);
  dict.SetMethod("getResourceUsage", &GetResourceUsage);
  dict.SetMethod("clearCache", &ClearCache);
  dict.SetMethod("_findFrameByRoutingId", &FindFrameByRoutingId);
  dict.SetMethod("_getFrameForSelector", &GetFrameForSelector);
  dict.SetMethod("_findFrameByName", &FindFrameByName);
  dict.SetMethod("_getOpener", &GetOpener);
  dict.SetMethod("_getParent", &GetFrameParent);
  dict.SetMethod("_getTop", &GetTop);
  dict.SetMethod("_getFirstChild", &GetFirstChild);
  dict.SetMethod("_getNextSibling", &GetNextSibling);
  dict.SetMethod("_getRoutingId", &GetRoutingId);
}
}  
NODE_LINKED_MODULE_CONTEXT_AWARE(atom_renderer_web_frame, Initialize)