Minor cleanup of GrResourceAllocator

This also adds a useful debug print out (of the end-of-opList array)

Change-Id: Ia678b6e8d143ca4f851984fb5026c2ec4f998384
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/202706
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>