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

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-repeat: repeat;
 background-size: auto;
@@ skipping 131 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 73 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%;