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When PSI memory pressure is high, triggers the moderate memory pressure.
Add feature flag CrOSLowMemoryNotificationPSI.

BUG=b:159299004
TEST=check moderate memory pressure is triggered properly when using \
    disk swap.

Change-Id: Iba533d5e4733619e2df7480eb2851b955b7ec99a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2428503
Commit-Queue: Kuo-Hsin Yang <vovoy@chromium.org>
Reviewed-by: Brian Geffon <bgeffon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837032}