Mark console-viewport-selection.html as flaky on Mac and XP.

BUG=390204
TBR=lushnikov@chromium.org,vsevik@chromium.org

Review URL: https://codereview.chromium.org/366883006

git-svn-id: svn://svn.chromium.org/blink/trunk@177571 bbb929c8-8fbe-4397-9dbb-9b2b20218538