SK_ARRAY_COUNT shouldn't work on pointers.

Simpler version of https://codereview.chromium.org/1123923002/

BUG=skia:3593
R=brucedawson@chromium.org,reed@google.com,tfarina@chromium.org

No public API changes.
TBR=reed@google.com

Review URL: https://codereview.chromium.org/1114283003