Remove 3-args |SVGAnimatedLength::baseValueToString()| impl.

This CL is to prepare for auto-dispatching baseValueToString in SVG*Element::parseAttribute.
SVGAnimatedLength required {Forbid,Allow}NegativeValues setting to be passed in as an argument of baseValueToString.
This was making auto-dispatch for baseValueToString calls difficult.

This CL removes the SVGLengthNegativeValuesMode argument from |SVGAnimatedLength::baseValueToString()|. The setting is passed in by |SVGAnimatedLength| ctor.

BUG=349370

Review URL: https://codereview.chromium.org/206133005

git-svn-id: svn://svn.chromium.org/blink/trunk@169648 bbb929c8-8fbe-4397-9dbb-9b2b20218538