Use access directly instead of sk_exists.

Skia would like to move SkOSFile and make it private spi. This appears
to be the only place SkOSFile is used outside Skia, and using access
here directly is more apropriate.

Test: Simple inline refactor, built and ran. Changes test code only.
Change-Id: Ib76c180bb3bccc54c20dcadc842837e3e8270929