fix(large-video): do not try to show background on safari with webrtc (#2606)

The animation for toggling filmstrip visibility was lagging on
Safari. Even though the background video is set to hidden, it is
still causing issues. Setting the background to display none
instead does help but might interfere with animations. So instead
do the easy thing and re-use logic used for Firefox to not show
the background video.

modules/UI/videolayout/VideoContainer.js

      * @returns {void}
      */
     _updateBackground() {
-        if (browser.isFirefox() || browser.isTemasysPluginUsed()) {
+        // Do not the background display on browsers that might experience
+        // performance issues from the presence of the background.
+        if (browser.isFirefox()
+                || browser.isSafariWithWebrtc()
+                || browser.isTemasysPluginUsed()) {
             return;
         }