Upgrade MediaElement to 2.15.0:

* It is required to set `height` on the parent `.wp-video` wrapper for video
* Fixes the play button hover problem in Chrome
* Includes my pull request: https://github.com/johndyer/mediaelement/pull/1273

All changes:
https://github.com/johndyer/mediaelement/compare/2.14.2...master

See #29110.

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