Bug 1550633 - part17 : modify test 'browser_cache.js'. r=timhuang

As now we won't automatically load disabled text track, we have to mark track as `default` in order to trigger loading.

Differential Revision: https://phabricator.services.mozilla.com/D32359

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

browser/components/originattributes/test/browser/browser_cache.js

@@ -184,6 +184,7 @@ async function doTest(aBrowser) {
       audioSource.setAttribute("type", "audio/ogg");
       audioTrack.setAttribute("src", trackURL);
       audioTrack.setAttribute("kind", "subtitles");
+      audioTrack.setAttribute("default", true);
 
       audio.appendChild(audioSource);
       audio.appendChild(audioTrack);