Create a general interface for getting platform-specific media keys event source in order to remove platform specific code from non-platform related folder `dom/media`.
Differential Revision: https://phabricator.services.mozilla.com/D55892
--HG--
rename : dom/media/mediacontrol/MediaHardwareKeysEventSourceMac.h => widget/cocoa/MediaHardwareKeysEventSourceMac.h
rename : dom/media/mediacontrol/MediaHardwareKeysEventSourceMac.mm => widget/cocoa/MediaHardwareKeysEventSourceMac.mm
extra : moz-landing-system : lando
Alastor Wu