Unskip SvgNestedUse.html

This test was skipped because long ago it crashed, however I can't reproduce that anymore.
Original bug is:
https://bugs.webkit.org/show_bug.cgi?id=100262

Review URL: https://codereview.chromium.org/295903004

git-svn-id: svn://svn.chromium.org/blink/trunk@174542 bbb929c8-8fbe-4397-9dbb-9b2b20218538