CSS Animations: Remove unnecessary webkit prefixes from layout tests

third_party/WebKit/LayoutTests/animations/checkbox-padding-animation-crash.html

 <script src="../resources/testharness.js"></script>
 <script src="../resources/testharnessreport.js"></script>
 <style>
-@-webkit-keyframes anim {
+@keyframes anim {
   to { padding-top: 50px; }
 }
-input { -webkit-animation: anim 1s; }
+input { animation: anim 1s; }
 </style>
 <input type="checkbox" id="target"></input>
 <script>
 test(function() {
   getComputedStyle(target);
 }, 'This test passes if it does not crash.')
 </script>