[Mac] media/track/track-mode.html sometimes trigger InvalidStateError
https://bugs.webkit.org/show_bug.cgi?id=115045

Reviewed by Alexey Proskuryakov.

* media/track/track-mode.html: Don't assume that the video element is ready for seeking
    after a 100ms timeout.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149197 268f45cc-cd09-0410-ab3c-d52691b4dbfc