Force layout to make a track rendering test non-flaky

While !textTracksAreReady(), readyState is constrained to
HAVE_CURRENT_DATA or lower. Once text tracks are ready, if the media
load has already progressed far, the canplaythrough event dispatch and
the text track rendering will be racy.

Since the rendering is not observable to Web content, it does not seem
worthwhile trying to make this deterministic.

The same fix was applied to WebKit:
http://trac.webkit.org/changeset/148050/trunk/LayoutTests/media/track/track-cue-container-rendering-position.html

BUG=330049

Review URL: https://codereview.chromium.org/533073003

git-svn-id: svn://svn.chromium.org/blink/trunk@181310 bbb929c8-8fbe-4397-9dbb-9b2b20218538