gardening: mark http/tests/inspector-enabled/target-leak.html is flaky on Win

BUG=390513
TBR=sergeyv

Review URL: https://codereview.chromium.org/368453004

git-svn-id: svn://svn.chromium.org/blink/trunk@177301 bbb929c8-8fbe-4397-9dbb-9b2b20218538