Switch back to mallinfo in SkSL memory benchmark

Early results from measuring RSS (via /proc or similar) shows results
that aren't very accurate or stable. From previous detailed testing, I
know that mallinfo gives a good answer. We only really need results
from ~one machine, so limiting to UNIX bots isn't a big deal.

Change-Id: I73af043720b1204e1da436e2e63b19766a97a9a2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/301445
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed