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The first memory system-health story ran in a set has been reporting
memory use differently all subsequent runs. This is due to how we
flush the system caches.

Just before we measure memory we flush the system caches unfortunately
this doesn't immediately take effect, instead the next story run is
effected. Due to this the first story run has anomalous results.
This option causes us to flush caches each time before Chrome starts
so we effect even the first story - avoiding the bug.

*************** Note to Perf Sheriff ****************

Regressions across several memory metrics are
expected for system_health.memory_mobile,
system_health.memory_desktop and memory.top10_mobile
as we stop under counting memory use.

*****************************************************

BUG=chromium:671156
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.perf:linux_perf_cq;master.tryserver.chromium.perf:mac_retina_perf_cq

Review-Url: https://codereview.chromium.org/2589173004
Cr-Commit-Position: refs/heads/master@{#443273}