Bug 1590780 - Cleanup PiP video listeners when closed r=mconley

Differential Revision: https://phabricator.services.mozilla.com/D50670

--HG--
extra : moz-landing-system : lando

toolkit/actors/PictureInPictureChild.jsm

@@ -1027,6 +1027,7 @@ class PictureInPictureChild extends ActorChild {
       "unload",
       () => {
         if (this.weakVideo) {
+          this.untrackOriginatingVideo(this.weakVideo);
           this.weakVideo.stopCloningElementVisually();
         }
         gWeakVideo = null;