Support viewport units for SVG Length animations

Add support of viewport units such as vm,vw, vmin, vmax for animations on SVG Length.
Chrome already supporting all viewport units for SVGLength.
https://bugs.chromium.org/p/chromium/issues/detail?id=368598
In that case, supporting those units for SVG animation will make sense.

BUG=626297

Review-Url: https://codereview.chromium.org/2130753003
Cr-Commit-Position: refs/heads/master@{#405038}
2 files changed