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

LayoutTests/transitions/multiple-background-transitions.html

Index: LayoutTests/transitions/multiple-background-transitions.html
diff --git a/LayoutTests/transitions/multiple-background-transitions.html b/LayoutTests/transitions/multiple-background-transitions.html
index b6b30dfe732a28ed6b1ab6ff29c1f1d5b0df6e32..f2cac383d8330783d840e9dfaeb424b4b26e3ccb 100644
--- a/LayoutTests/transitions/multiple-background-transitions.html
+++ b/LayoutTests/transitions/multiple-background-transitions.html
@@ -50,9 +50,9 @@
 
     const expectedValues = [
       // [time, element-id, property, expected-value, tolerance]
-      [0.5, 'box', 'background-position', [40, 40], 2],
-      [0.5, 'box2', 'background-position', [40, 40], 2],
-      [0.5, 'box3', 'background-position', [40, 40], 2],
+      [0.5, 'box',  'background-position', ['left', 40, 'top', 40], 2],
+      [0.5, 'box2', 'background-position', ['left', 40, 'top', 40], 2],
+      [0.5, 'box3', 'background-position', ['left', 40, 'top', 40], 2],
     ];
   
     function setupTest()