Mark http/tests/security/frame-loading-via-document-write.html as flaky

Started failing after r157475, which modified the test.

BUG=289002
TBR=eseidel

Review URL: https://chromiumcodereview.appspot.com/23432011

git-svn-id: svn://svn.chromium.org/blink/trunk@157554 bbb929c8-8fbe-4397-9dbb-9b2b20218538