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Use memory_instrumentation::OSMetrics::FillOSMemoryDump(), which
reads /proc/<pid>/statm, to get private bytes of a process, to avoid
contention with kswapd under heavy memory pressure.

BUG=chromium:872253
TEST=manual
R=cylee@chromium.org, sonnyrao@chromium.org, fdoray@chromium.org

Change-Id: I4d43933a39c3c89d8ebb81e3ccef20277cedb258
Reviewed-on: https://chromium-review.googlesource.com/1212246
Commit-Queue: Chinglin Yu <chinglinyu@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Cheng-Yu Lee <cylee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592701}