[CSS Grid Layout] Adapting align-self, align-items and justify-self to the last CSS 3 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 31 matching lines @@
 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: 100px
 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: 0px
 margin-left: auto
 margin-right: 0px
 margin-top: 0px
@@ 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 88 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%