Add toSVGColor|Paint() to cleanup static_cast<>, and use it

SVGColor and SVGPaint can't use DEFINE_CSS_VALUE_TYPE_CASTS(), this cl adds
toSVGColor() and toSVGPaint() manually. Those functions will help to detect
bad type cast.

BUG=N/A

Review URL: https://codereview.chromium.org/26261009

git-svn-id: svn://svn.chromium.org/blink/trunk@159425 bbb929c8-8fbe-4397-9dbb-9b2b20218538