Add the MediaSessionAction enum (#6488)
Add the MediaSessionAction enum (#6488)

See also https://github.com/mdn/browser-compat-data/issues/6265

This adds this string enum with the description "Media Session
action types". It defines the strings that identify the types
of actions to be handled by a media session.

- Chrome
* `seekto`
  * Bug: https://bugs.chromium.org/p/chromium/issues/detail?id=977375
  * [Commit maps to version 77](https://storage.googleapis.com/chromium-find-releases-static/f23.html#f230e6257d8bd8637314b5dd4d7acb4ae997f604)
* `skipad`
  * Bug: https://bugs.chromium.org/p/chromium/issues/detail?id=910436
  * [Commit maps to version 73](https://storage.googleapis.com/chromium-find-releases-static/b02.html#b0281b1250cdec351658e9bf07dd5f5d3331ef91)

- Firefox
* Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1580623

Other data is taken from the existing documentation and BCD for the
rest of the Media Session API.
chrisdavidmills/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data,mdn/browser-compat-data