Add memuse stats and fix disk I/O computations.

This changes adds memory usage stats after 1  5, 30, 150, and 750
minutes of active use (approximated by wall
clock time minus sleep time).  We log only the anonymous memory
usage (active + inactive) as we expect that other types of
allocations would not convey much additional information.

This also fixes the disk I/O computation to use actual active
time elapsed between callbacks, instead of assuming the expected
interval has elapsed.  The existing code causes errors in
both directions when a suspend/resume cycle occurs between callbacks.

BUG=14209
TEST=verify that about:histograms page contains Platform.MemuseAnon1 after 1 minute and Platform.MemuseAnon2 after 5 minutes.

Change-Id: Ib32d915fac7766a9fca7125105224889ea93050e
Reviewed-on: http://gerrit.chromium.org/gerrit/1132


Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Luigi Semenzato <semenzato@chromium.org>