Use Length for the stroke-dashoffset property in SVGLayoutStyle

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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 color-rendering: auto;
 fill: rgb(0, 0, 0);
 fill-opacity: 1;
 fill-rule: nonzero;
 marker-end: none;
 marker-mid: none;
 marker-start: none;
 shape-rendering: auto;
 stroke: none;
 stroke-dasharray: none;
-stroke-dashoffset: 0;
+stroke-dashoffset: 0px;
 stroke-linecap: butt;
 stroke-linejoin: miter;
 stroke-miterlimit: 4;
 stroke-opacity: 1;
 stroke-width: 1;
 alignment-baseline: auto;
 baseline-shift: 0px;
 dominant-baseline: auto;
 text-anchor: start;
 writing-mode: lr-tb;
 glyph-orientation-horizontal: 0deg;
 glyph-orientation-vertical: auto;
 vector-effect: none;
 
 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%;