Fix assert in SVGRenderStyle.cpp due to not considering visted links

This is a merge of http://trac.webkit.org/changeset/153914 by Rob Buis.

Previously the style diff code did not consider visited links in
SVGRenderStyle.cpp. This patch updates the diff calculation to include
visited links.

BUG=271259

Review URL: https://chromiumcodereview.appspot.com/22652009

git-svn-id: svn://svn.chromium.org/blink/trunk@155943 bbb929c8-8fbe-4397-9dbb-9b2b20218538