MallocPixelRef should always allocate as large as computeByteSize() says

Bug: 1103827
Change-Id: I837f92cf10a1a389fe1b0ba55ae1323e7e68f741
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/304416
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Reed <reed@google.com>
(cherry picked from commit c1eb58de32c016eb60e7a46046321ffe351e8222)
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/308758
Reviewed-by: Heather Miller <hcm@google.com>