fast/dom/custom/leaks.html is no longer flaky => remove expectation.

Perhaps it was correctly detecting a flaky leak.

BUG=269664
TBR=fmalita@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22880016

git-svn-id: svn://svn.chromium.org/blink/trunk@156277 bbb929c8-8fbe-4397-9dbb-9b2b20218538