Avoid unnecessary vector copy in AnimationController event dispatch.
<https://webkit.org/b/122994>

Use Vector's move constructor instead of making a copy of the pending
events queue and then clearing it.

Reviewed by Simon Fraser.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@157704 268f45cc-cd09-0410-ab3c-d52691b4dbfc