Linux: use /proc/pid/statm to get memory stats when /proc/pid/smaps is unavailable.

BUG=23258
TEST=We get approximate memory usage in about:memory for sandboxed renderers.
Review URL: http://codereview.chromium.org/365007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@31019 0039d316-1c4b-4281-b951-d872f2087c98