tweak SkArenaAlloc comments, parameter names

Change-Id: Ie7518f2ad2e0f11f0acd5a3bfeff4e5ca7188311
Reviewed-on: https://skia-review.googlesource.com/132925
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Herb Derby <herb@google.com>
1 file changed