Use 4 value syntax for getComputedStyle of background-position and -webkit-mask-position

LayoutTests/transitions/multiple-mask-transitions.html

Index: LayoutTests/transitions/multiple-mask-transitions.html
diff --git a/LayoutTests/transitions/multiple-mask-transitions.html b/LayoutTests/transitions/multiple-mask-transitions.html
index e4bd3519c9927b3f76c67f880d9021e6105022f7..6fc52b7e67e62a09e36d4e4340198e9369d801d5 100644
--- a/LayoutTests/transitions/multiple-mask-transitions.html
+++ b/LayoutTests/transitions/multiple-mask-transitions.html
@@ -50,9 +50,9 @@
 
     const expectedValues = [
       // [time, element-id, property, expected-value, tolerance]
-      [0.5, 'box', '-webkit-mask-position', [40, 40], 5],
-      [0.5, 'box2', '-webkit-mask-position', [40, 40], 5],
-      [0.5, 'box3', '-webkit-mask-position', [40, 40], 5],
+      [0.5, 'box',  '-webkit-mask-position', ['left', 40, 'top', 40], 5],
+      [0.5, 'box2', '-webkit-mask-position', ['left', 40, 'top', 40], 5],
+      [0.5, 'box3', '-webkit-mask-position', ['left', 40, 'top', 40], 5],
     ];
   
     function setupTest()