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We currently use statm on linux. This provides less than great results.
We take total resident memory and subtract "shared pages" from it instead of
using a count of pages that only this process has mapped.

On chromeos we've added totmaps which sums up the fields from the process'
smaps.

BUG=125150
TEST=Examine memory reports from about:memory and task manager, and compare
     them to the sums of the results in the smaps.

R=thestig@chromium.org, willchan@chromium.org

Review URL: https://codereview.chromium.org/15779007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@202639 0039d316-1c4b-4281-b951-d872f2087c98