Fix base URL in mediasession mediametadata test

Change-Id: Ia25e9daf97e980b2e3ce9a1ba92b8f3bb5d28239
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4301375
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Reviewed-by: Fr <beaufort.francois@gmail.com>
Commit-Queue: Fr <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/main@{#1112881}

mediasession/mediametadata.html

@@ -212,7 +212,7 @@
 
   assert_equals(artwork.length, URLs.length);
   for (let i = 0 ; i < artwork.length ; ++i) {
-    assert_equals(artwork[i].src, new URL(URLs[i], document.URL).href);
+    assert_equals(artwork[i].src, new URL(URLs[i], subframe.contentDocument.URL).href);
   }
 }, 'Test that the base URL of MediaImage is the base URL of entry setting object');