CSS Animations: Fix SVG visited color interpolation

SVG paint interpolations were not taking into account the visited link
colors. This caused transitions on visited colors to not interpolate
smoothly.
This patch adds visited link interpolation to AnimatableSVGPaint
similar to AnimatableColor.

BUG=314892

Review URL: https://codereview.chromium.org/291173002

git-svn-id: svn://svn.chromium.org/blink/trunk@174666 bbb929c8-8fbe-4397-9dbb-9b2b20218538