[CSS Grid Layout] Upgrade justify-content parsing to CSS3 Box Alignment spec.

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-repeat: repeat
 background-size: auto
 border-bottom-color: rgb(0, 0, 0)
@@ skipping 130 matching lines @@
 -webkit-column-rule-color: rgb(0, 0, 0)
 -webkit-column-rule-style: none
 -webkit-column-rule-width: 0px
 -webkit-column-span: none
 -webkit-column-width: auto
 align-content: stretch
 align-items: start
 align-self: start
 flex-direction: row
 flex-wrap: nowrap
-justify-content: flex-start
+justify-content: start
 -webkit-font-smoothing: auto
 -webkit-highlight: none
 -webkit-hyphenate-character: auto
 -webkit-line-box-contain: block inline replaced
 -webkit-line-break: auto
 -webkit-line-clamp: none
 -webkit-locale: auto
 -webkit-margin-before-collapse: collapse
 -webkit-margin-after-collapse: collapse
 -webkit-mask-box-image: none
@@ skipping 85 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%