always zero SkMallocPixelRefs

I'm getting tired of trying to figure out where
clients screw up and forget to clear these buffers,
and I'd like a safer safety net for our own screw ups.

Bug: chromium:934161, many more
Change-Id: I6ada4c821da6dd173e54c6402c17d6946ff05fdf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/207857
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
6 files changed