Reenable test marked as flaky

Unable to repro locally and disabling too old: no information about flakiness.

BUG=368531

Review URL: https://codereview.chromium.org/632353004

git-svn-id: svn://svn.chromium.org/blink/trunk@183483 bbb929c8-8fbe-4397-9dbb-9b2b20218538