[SVG] Move negative viewBox width/height reporting to SVGAnimatedViewBoxRect

Before this CL, SVGFitToViewBox::parseAttribute handled negative width, height
errors inline. To migrate parsing to property map (crbug.com/397902), we want
to delegate this to SVGAnimatedProperty::setBaseValueToString implementation.

This CL introduces SVGAnimatedViewBoxRect which is a SVGAnimatedRect implements
handling of negative width/height.

TEST=svg/custom/viewbox-syntax.svg
BUG=397902

Review URL: https://codereview.chromium.org/664433002

git-svn-id: svn://svn.chromium.org/blink/trunk@183802 bbb929c8-8fbe-4397-9dbb-9b2b20218538