Use Length for baselineShiftValue in SVGLayoutStyle

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 207 matching lines @@
 shape-rendering: auto
 stroke: none
 stroke-dasharray: none
 stroke-dashoffset: 0
 stroke-linecap: butt
 stroke-linejoin: miter
 stroke-miterlimit: 4
 stroke-opacity: 1
 stroke-width: 1
 alignment-baseline: auto
-baseline-shift: baseline
+baseline-shift: 0px
 dominant-baseline: auto
 text-anchor: start
 writing-mode: lr-tb
 glyph-orientation-horizontal: 0deg
 glyph-orientation-vertical: auto
 vector-effect: none
 background-position-x: 0%
 background-position-y: 0%
 border-spacing: 0px 0px
 overflow: visible
 -webkit-mask-position-x: 0%
 -webkit-mask-position-y: 0%
 
 After changing grandparent's 'color' and the parent's 'font-size' inherited properties:
 border-top-width: 18px
 color: rgb(0, 0, 255)
 
 After changing the parent's 'display' property to 'block', so that the child gets a renderer:
 height: 100px
 margin-left: 350px
 width: 350px
 
 After changing the parent's 'display' property back to 'none':
 height: 100px
 margin-left: auto
 width: 50%