Work around GC bug to fix a flaky AudioTrackList/VideoTrackList test

BUG=397324
BUG=400659

Review URL: https://codereview.chromium.org/440033003

git-svn-id: svn://svn.chromium.org/blink/trunk@179558 bbb929c8-8fbe-4397-9dbb-9b2b20218538