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This CL creates the SystemMemoryPressureEvaluator for Windows, as part
of a refactoring of the MemoryPressureMonitor to use the
Voter-Aggregator design described here:
https://docs.google.com/document/d/1W3FPDyjIAKBcFGNYsHA3EKR1FHrJlbBaqT4_RUnxzq0/edit?ts=5d3f5714#heading=h.7nki9mck5t64
This Evaluator calculates memory pressure and emits notifications
identically to the previous Monitor, but does so by casting its votes
for memory pressure through a MemoryPressureVoter.

Bug: 980965
Change-Id: Ib2fe6e7699762c209411a08ea1c938998bffe641
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1721376Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Ryan Powell <ryanpow@google.com>
Cr-Commit-Position: refs/heads/master@{#688273}