Negative SVGTransform scale values should be correctly stringified

Before this CL, |SVGTransform::valueAsString()| used |AffineTransform::{x,y}Scale}|, which computed transform scale values by computing norm of projected unit vectors. However, that would always return positive values, even if negative scale values were specified.

This CL changes the implementation to use |a| and |d| values instead.
This is valid, as the scale matrix is guaranteed to be in the below simple form when the transform has type |SVG_TRANSFORM_SCALE|:
|xscale    0   0|
|   0   yscale 0|

BUG=393917

Review URL: https://codereview.chromium.org/410573002

git-svn-id: svn://svn.chromium.org/blink/trunk@178633 bbb929c8-8fbe-4397-9dbb-9b2b20218538