Merge branch 'use-default-closest-if-available' into 'master'

Use default `closest` if available!

## What does this MR do?

We should use the default `Element.prototype.closest` if its available.

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [ ] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

See merge request !7706

app/assets/javascripts/extensions/element.js.es6

@@ -3,7 +3,7 @@
 
 Element.prototype.matches = Element.prototype.matches || Element.prototype.msMatchesSelector;
 
-Element.prototype.closest = function closest(selector, selectedElement = this) {
+Element.prototype.closest = Element.prototype.closest || function closest(selector, selectedElement = this) {
   if (!selectedElement) return;
   return selectedElement.matches(selector) ? selectedElement : Element.prototype.closest(selector, selectedElement.parentElement);
 };