[CSS Grid Layout] Adapting align-self, align-items and justify-self to the last CSS 3 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;
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 empty-cells: show;
 float: none;
 font-kerning: auto;
 font-size: 16px;
 font-style: normal;
 font-variant: normal;
 font-variant-ligatures: normal;
 font-weight: normal;
 height: 576px;
 image-rendering: auto;
+justify-items: start;
+justify-self: start;
 left: auto;
 letter-spacing: normal;
 line-height: normal;
 list-style-image: none;
 list-style-position: outside;
 list-style-type: disc;
 margin-bottom: 8px;
 margin-left: 8px;
 margin-right: 8px;
 margin-top: 8px;
@@ skipping 74 matching lines @@
 -webkit-column-break-before: auto;
 -webkit-column-break-inside: auto;
 -webkit-column-count: auto;
 -webkit-column-gap: normal;
 -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: stretch;
-align-self: stretch;
+align-items: start;
+align-self: start;
 flex-direction: row;
 flex-wrap: nowrap;
 justify-content: flex-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;
@@ skipping 76 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%;