turn on hidden visibility flags on iOS

turn on hidden visibility flags on iOS

These new images came up when I turned them off the other day:

https://gold.skia.org/search?blame=eec23d113149569414e325ac6fd5a9253945628b&fdiffmax=-1&fref=false&frgbamax=-1&head=true&include=false&limit=50&match=gamma_correct&match=name&metric=combined&neg=false&pos=false&query=source_type%3Dgm&sort=desc&unt=true

Some sort of ODR problem?  :/

Change-Id: I9b2575948c3bd5710476e6501e8ec27933c277f6
Reviewed-on: https://skia-review.googlesource.com/10287
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>