mark xss-DENIED-to-data-url tests as even more flaky.

TBR=
BUG=372425,352464

Review URL: https://codereview.chromium.org/291063003

git-svn-id: svn://svn.chromium.org/blink/trunk@174332 bbb929c8-8fbe-4397-9dbb-9b2b20218538