D28936: Use a Proxy object to detect changes within the MediaMetadata

[Kai Uwe Broulik]

broulik created this revision.
broulik added reviewers: Plasma, fvogt.
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REVISION SUMMARY
  According to spec [1] it is allowed to change properties within the `MediaMetadata` rather than assigning a new instance.
  Using a `Proxy` object lets us detect property assignments within the object. Conveniently, it still reports `instanceof` as being the original object.
  
  [1] https://www.w3.org/TR/mediasession/#the-mediametadata-interface (last sentence)

TEST PLAN
  This fixes VKontakte player not updating metadata when changing tracks. Thanks @IlyaBizyaev for investigating.
  
  YouTube, vimeo, media session example (who all set new metadata instance when they change) still get their metadata propagated correctly

REPOSITORY
  R856 Plasma Browser Integration

REVISION DETAIL
  https://phabricator.kde.org/D28936

AFFECTED FILES
  extension/content-script.js

To: broulik, #plasma, fvogt
Cc: IlyaBizyaev, plasma-devel, Orage, LeGast00n, The-Feren-OS-Dev, cblack, jraleigh, zachus, fbampaloukas, ragreen, ZrenBot, ngraham, himcesjf, lesliezhai, ali-mohamed, jensreuterberg, abetts, sebas, apol, ahiemstra, mart
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