fix(Ads): Fix interstitials support when using PiP on Safari (#8875)

Author: avelad

lib/ads/interstitial_ad_manager.js

@@ -323,7 +323,8 @@ shaka.ads.InterstitialAdManager = class {
     let supportsMultipleMediaElements =
         this.config_.supportsMultipleMediaElements;
     const video = /** @type {HTMLVideoElement} */(this.baseVideo_);
-    if (video.webkitSupportsFullscreen && video.webkitDisplayingFullscreen) {
+    if (video.webkitPresentationMode &&
+        video.webkitPresentationMode !== 'inline') {
       supportsMultipleMediaElements = false;
     }
     if (this.usingBaseVideo_ != supportsMultipleMediaElements) {