Fix duplicate timeupdate events

This change prevents duplicate timeupdate events caused by the text
track code. The comment above the call indicates that the timeupdate
event should have the same restrictions as the normal periodic
timeupdate event. By changing the parameter passed to
scheduleTimeupdateEvent() to true, the event rate limiting guards
are properly applied to this call and duplicate events won't
fire.

TEST=Manually tested that timeupdate events don't fire more
often than every 250ms during normal playback. An automated
test to verify this would likely be flaky.

Review URL: https://codereview.chromium.org/108253006

git-svn-id: svn://svn.chromium.org/blink/trunk@163465 bbb929c8-8fbe-4397-9dbb-9b2b20218538