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

LayoutTests/fast/css/getComputedStyle/computed-style-without-renderer-expected.txt

 Computed style of an element whose parent's 'display' value is 'none':
 background-attachment: scroll
 background-clip: border-box
 background-color: rgb(0, 128, 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: 50% 50%
 -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 66 matching lines @@
 height: 100px
 margin-left: 350px
 width: 350px
 
 After changing the parent's 'display' property back to 'none':
 height: 100px
 margin-left: auto
 width: 50%