Mark screen_orientation/page-visibility.html as flaky.

TBR=ch.dumez@samsung.com,mlamouri@chromium.org
BUG=389047
NOTRY=true

Review URL: https://codereview.chromium.org/336863006

git-svn-id: svn://svn.chromium.org/blink/trunk@176973 bbb929c8-8fbe-4397-9dbb-9b2b20218538