Bug 1664899 - Make sure the experimental toggle element is hidden when in fullscreen. r=Gijs

Differential Revision: https://phabricator.services.mozilla.com/D90164

toolkit/themes/shared/media/videocontrols.css

@@ -71,7 +71,8 @@
 .controlBar[hidden] .progressBar,
 .controlBar[hidden] .bufferBar,
 .pictureInPictureToggleButton[hidden],
-.videocontrols[inDOMFullscreen] > .controlsContainer > .controlsOverlay > .pictureInPictureToggleButton {
+.videocontrols[inDOMFullscreen] > .controlsContainer > .controlsOverlay > .pictureInPictureToggleButton,
+.videocontrols[inDOMFullscreen] > .controlsContainer > .controlsOverlay > #pictureInPictureToggleExperiment {
   display: none;
 }