alwu 67f934d684 Bug 1591620 - part2 : allow more customers to moniter media keys event by adding a listener to the event source. r=chunmin
Create methods to add or remove listener directly to the media control keys event source.

So if anyone has a need to monitor media control keys event, they can simply implement their own listener and use `MediaControlService::GetMediaControlKeysManager()` to add or remove a listener.

Differential Revision: https://phabricator.services.mozilla.com/D50687

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extra : moz-landing-system : lando
2019-10-30 00:50:44 +00:00
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