Bug 1636204 - Use correct indexes for parsing /proc/[pid]/stat. r=Yoric

All the indexes were off by two, so the values were for completely different
measurements, and thus nonsense. Resident Set Size still doesn't seem to be
handled correctly, perhaps because it's being treated as a bytes count rather
than the page count it is somewhere, but with this change the Virtual Memory
figures match what system monitor displays.

Differential Revision: https://phabricator.services.mozilla.com/D74349