Fixed bug where animation time was not updated when dynamically loading a style sheet
        https://bugs.webkit.org/show_bug.cgi?id=37352

        A dynamically loaded style sheet kicks off its own styleRecalc cycle. This was not
        bracketed with a beginAnimationUpdate/endAnimationUpdate which wasn't resetting the
        animation time. In some time-dependent cases this was causing a negative elapsedTime
        to be sent to the keyframe animator. This is an invalid case which destroys the
        animation prematurely. I not only added the brackets, but I also added an assert
        and protection for when the elapsedTime comes up negative.

        Test: animations/dynamic-stylesheet-loading.html

        * dom/Document.cpp: Added brackets
        (WebCore::Document::updateStyleSelector):
        * page/animation/KeyframeAnimation.cpp: Added assert and protection
        (WebCore::KeyframeAnimation::getKeyframeAnimationInterval):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@57390 268f45cc-cd09-0410-ab3c-d52691b4dbfc