Mark more timeline tests and an encrypted-media test as flaky.

BUG=407915,407993
TBR=hclam@chromium.org

Review URL: https://codereview.chromium.org/530913002

git-svn-id: svn://svn.chromium.org/blink/trunk@181212 bbb929c8-8fbe-4397-9dbb-9b2b20218538