Minor code cleanup in AnimationBase::progress
https://bugs.webkit.org/show_bug.cgi?id=114371

Reviewed by Dean Jackson.

Convert some "return; else if" to just "return; if".

* page/animation/AnimationBase.cpp:
(WebCore::AnimationBase::progress):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148122 268f45cc-cd09-0410-ab3c-d52691b4dbfc