move zero-init to sk_malloc for masks

Bug: skia:
Change-Id: I75d557068bdcd9e9f7e380e4fa447f9d83dd1554
Reviewed-on: https://skia-review.googlesource.com/98200
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Reed <reed@google.com>
5 files changed