Add error handling and /proc/statm count in multi-process-rss.py.

BUG=324194
TEST=None
NOTRY=True

Review URL: https://codereview.chromium.org/106973006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240119 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240119