Fix backface-visibility-hidden-001.html to stop relying on unitless values for perspective (#31522) 

WebKit export of https://bugs.webkit.org/show_bug.cgi?id=104805
diff --git a/css/css-transforms/backface-visibility-hidden-001.html b/css/css-transforms/backface-visibility-hidden-001.html
index f3b5a75..2476f5b 100644
--- a/css/css-transforms/backface-visibility-hidden-001.html
+++ b/css/css-transforms/backface-visibility-hidden-001.html
@@ -41,7 +41,7 @@
         .container {
             width: 200px;
             height: 200px;
-            perspective: 1000;
+            perspective: 1000px;
             transform: rotateY(45deg);
         }