DevTools: hide transiently enabled experiments on the settings screen

We don't allow to disable transiently enabled experiments so they shouldn't be present on the settings screen.

BUG=None

Review URL: https://codereview.chromium.org/619883002

git-svn-id: svn://svn.chromium.org/blink/trunk@183057 bbb929c8-8fbe-4397-9dbb-9b2b20218538