Unprefix usage of -webkit-transform in tests

LayoutTests/compositing/squashing/resources/squash-with-ancestor-property.css

 #composited-background {
   width: 100%;
   height: 100%;
-  -webkit-transform: translateZ(0);
+  transform: translateZ(0);
 }
 
 #container {
   position: relative;
   width: 400px;
   height: 60px;
   border: 1px solid black;
 }
 
 #composited-overlap-child {
   position: absolute;
   left: 0;
   bottom: 0px;
   width: 100%;
   height: 60px;
   background-color: orange;
 }