D20726: Call into native Media Session browser API if available

[Kai Uwe Broulik]

broulik created this revision.
broulik added reviewers: Plasma, fvogt.
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Herald added a subscriber: plasma-devel.
broulik requested review of this revision.

REVISION SUMMARY
  Since Chrome 73 it also supports Media Session also on the desktop.
  The calls are re-arranged since `setActionHandler` throws if passed an invalid action and in this case we don't want to register the callback either.
  Also, there's a new "skipad" action for a "Skip Ad" overlay in picture-in-picture mode [1] for which to work we need to call the native browser APIs.
  Also, since `MediaMetadata` is a proper class and not an `Object`, iterate its prototype rather than just stringifying it and adjust our shim to do the same.
  
  [1] https://www.chromestatus.com/feature/4749278882824192

TEST PLAN
  Tried this example [1] in Chrome, now get a "Skip ad" button in the PIP overlay.
  Firefox still works as before
  
  [1] https://googlechrome.github.io/samples/picture-in-picture/skip-ad.html

REPOSITORY
  R856 Plasma Browser Integration

REVISION DETAIL
  https://phabricator.kde.org/D20726

AFFECTED FILES
  extension/content-script.js

To: broulik, #plasma, fvogt
Cc: plasma-devel, jraleigh, GB_2, ragreen, Pitel, ZrenBot, lesliezhai, ali-mohamed, jensreuterberg, abetts, sebas, apol, mart
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