[Gardening] Mark fast/dom/custom/leaks.html as flaky

BUG=269664
TBR=pkasting@chromium.org

Review URL: https://codereview.chromium.org/22605004

git-svn-id: svn://svn.chromium.org/blink/trunk@155704 bbb929c8-8fbe-4397-9dbb-9b2b20218538