Bug 1548446 - part1 : add more debug log in 'test_webvtt_seeking.html'. r=jolin

Add more log in order to help debug and know where we get failure.

Differential Revision: https://phabricator.services.mozilla.com/D30953

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

dom/media/test/test_webvtt_seeking.html

@@ -84,6 +84,7 @@ async function startVideo(video) {
 }
 
 async function waitUntilCueIsShowing(cue) {
+  info(`wait until cue ${cue.id} shows`);
   // cue has not been showing yet.
   if (!cue.getActive) {
     await once(cue, "enter");
@@ -98,6 +99,7 @@ async function seekVideo(video, time) {
   info(`seek video to ${time}`);
   video.currentTime = time;
   await once(video, "seeked");
+  info(`seek succeeded, current time=${video.currentTime}`);
 }
 
 function isInRange(value, lowerBound, higherBound) {