refactor SkArenaAlloc arrays a little

Change-Id: I746424ea6bf324e9841161c28a94527e07803653
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/289825
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed