Mark some tests flaky

BUG=356991, 356992, 356994, 356995, 356996, 356997
TBR=enne@chromium.org

Review URL: https://codereview.chromium.org/214213002

git-svn-id: svn://svn.chromium.org/blink/trunk@170127 bbb929c8-8fbe-4397-9dbb-9b2b20218538