Mark target-leak.html as flaky on more platforms.

BUG=390513
TBR=sergeyv@chromium.org,lushnikov@chromium.org

Review URL: https://codereview.chromium.org/375763002

git-svn-id: svn://svn.chromium.org/blink/trunk@177628 bbb929c8-8fbe-4397-9dbb-9b2b20218538