Don't update the text track override CSS every time a media element is created.
<http://webkit.org/b/119199>
<rdar://problem/14572855>

Reviewed by Darin Adler.

We should only have to update the CSS if the accessibility caption preferences are changed.
This was forcing a full style recalc in every Document any time a media element is instantiated.

* page/CaptionUserPreferencesMediaAF.cpp:
(WebCore::CaptionUserPreferencesMediaAF::setInterestedInCaptionPreferenceChanges):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@153418 268f45cc-cd09-0410-ab3c-d52691b4dbfc