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As per SVG2, SVGLength.value setter should set the
SVGLength's value to a <number> whose value is value.
https://svgwg.org/svg2-draft/types.html#__svg__SVGLength__value

This patch is supportive patch for calc() support for SVGLength.

BUG=558304

Review-Url: https://codereview.chromium.org/2113943003
Cr-Commit-Position: refs/heads/master@{#403440}