The reason we do not support OPENTYPE_VERTICAL on Mac yet is because no one has implemented it, not because it cannot be supported on that platform. Add this rationale to features.gypi to note that the guard should stay an ENABLE() guard.

BUG=286578

Review URL: https://codereview.chromium.org/112913003

git-svn-id: svn://svn.chromium.org/blink/trunk@164443 bbb929c8-8fbe-4397-9dbb-9b2b20218538