[DevTools] Settings: disallow editing overrides when they aren't enabled

Also disable "Enable Overrides on DevTools start-up".

BUG=271446

Review URL: https://chromiumcodereview.appspot.com/23049002

git-svn-id: svn://svn.chromium.org/blink/trunk@156009 bbb929c8-8fbe-4397-9dbb-9b2b20218538