Gardening: Mark some tests flaky.

BUG=401377,401380,401381
TBR=jbroman@chromium.org

Review URL: https://codereview.chromium.org/448893005

git-svn-id: svn://svn.chromium.org/blink/trunk@179687 bbb929c8-8fbe-4397-9dbb-9b2b20218538