[Fullscreen] Add a site-specific hack to work around "flash on exit" behavior of YouTube.com.
https://bugs.webkit.org/show_bug.cgi?id=136604

Reviewed by Eric Carlson.

YouTube.com will cause a "flash" of the full screen sized <video> element upon exiting full
screen because the "fullscreenchange" event is fired asynchronously after the exit animation
completes. Only YouTube sites and embeds, add a site-specific-quirk which runs the "fullscreenchange"
event synchronously at the end of the exit animation. This causes YouTube's video resizing logic
to run during the period of time where we've disabled screen updates, instead of immediately
after.

* dom/Document.cpp:
(WebCore::Document::webkitDidExitFullScreenForElement):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@173365 268f45cc-cd09-0410-ab3c-d52691b4dbfc