Gardening: Mark svg/animations/smil-leak-dynamically-added-element-instances.svg as flaky

This test seems to flay fail. The test was added in
https://bugs.webkit.org/show_bug.cgi?id=83856

TBR=pdr@chromium.org
BUG=377944

Review URL: https://codereview.chromium.org/302463008

git-svn-id: svn://svn.chromium.org/blink/trunk@174885 bbb929c8-8fbe-4397-9dbb-9b2b20218538