Allow fragment navigation from <svg:a>

This essentially reverts the fix for WK bug #19995 while adding a test for
the (new) expected behavior.

BUG=296071

Review URL: https://codereview.chromium.org/249313005

git-svn-id: svn://svn.chromium.org/blink/trunk@172722 bbb929c8-8fbe-4397-9dbb-9b2b20218538