Unmark http/tests/workers/terminate-during-sync-operation.html as flaky.

BUG=339539
TBR=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/134193013

git-svn-id: svn://svn.chromium.org/blink/trunk@166389 bbb929c8-8fbe-4397-9dbb-9b2b20218538