Mark plugins/plugin-clip-subframe.html as flaky for linux and lion release.

BUG=248063
TBR=hclam@chromium.org

Review URL: https://codereview.chromium.org/23249003

git-svn-id: svn://svn.chromium.org/blink/trunk@156193 bbb929c8-8fbe-4397-9dbb-9b2b20218538