Log 'waitingforkey' events in EME browser tests

Bug: 987067
Test: EME content_browsertests pass
Change-Id: Ia1573232a0c43b347e26ea4707406b92c652a82d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1719452Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: John Rummell <jrummell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681142}

media/test/data/eme_player.html

@@ -138,6 +138,7 @@
       video.addEventListener('canplaythrough', onLogEvent);
       video.addEventListener('stalled', onLogEvent);
       video.addEventListener('waiting', onLogEvent);
+      video.addEventListener('waitingforkey', onLogEvent);
       document.addEventListener('visibilitychange', onVisibilityChange);
       Utils.resetTitleChange();
       if (playTwice) {