Remove dangerous constructor from SkArenaAlloc

Make the constructors for SkArenaAlloc unambiguous.

TBR=mtklein@google.com

Change-Id: Iaa6419574e78e17bf746f5f9bcdf99c9d6c9d727
Reviewed-on: https://skia-review.googlesource.com/13540
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed