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

LayoutTests/fast/css/getComputedStyle/computed-style-expected.txt

 Attributes that are exposed in the CSS computed style object:
 
 background-attachment: scroll;
 background-clip: border-box;
 background-color: rgba(0, 0, 0, 0);
 background-image: none;
 background-origin: padding-box;
-background-position: 0% 0%;
+background-position: left 0% top 0%;
 background-repeat: repeat;
 background-size: auto;
 border-bottom-color: rgb(0, 0, 0);
 border-bottom-left-radius: 0px;
 border-bottom-right-radius: 0px;
 border-bottom-style: none;
 border-bottom-width: 0px;
 border-collapse: separate;
 border-image-outset: 0px;
 border-image-repeat: stretch;
@@ skipping 146 matching lines @@
 -webkit-mask-box-image: none;
 -webkit-mask-box-image-outset: 0px;
 -webkit-mask-box-image-repeat: stretch;
 -webkit-mask-box-image-slice: 0 fill;
 -webkit-mask-box-image-source: none;
 -webkit-mask-box-image-width: auto;
 -webkit-mask-clip: border-box;
 -webkit-mask-composite: source-over;
 -webkit-mask-image: none;
 -webkit-mask-origin: border-box;
--webkit-mask-position: 0% 0%;
+-webkit-mask-position: left 0% top 0%;
 -webkit-mask-repeat: repeat;
 -webkit-mask-size: auto;
 order: 0;
 -webkit-perspective: none;
 -webkit-perspective-origin: 392px 288px;
 -webkit-print-color-adjust: economy;
 -webkit-rtl-ordering: logical;
 -webkit-text-combine: none;
 -webkit-text-decorations-in-effect: none;
 -webkit-text-emphasis-color: rgb(0, 0, 0);
@@ skipping 54 matching lines @@
 
 Other attributes that the computed style class supports:
 
 background-position-x: 0%;
 background-position-y: 0%;
 border-spacing: 0px 0px;
 overflow: visible;
 -webkit-mask-position-x: 0%;
 -webkit-mask-position-y: 0%;