Added a flag, process_size, to collect the maximum memory size used by
a test run.

Bug: 
Change-Id: I5d659ff03bfcf7ab4a868aafbda3303ffcb9fa78
Reviewed-on: https://chromium-review.googlesource.com/542415
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46713}