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This CL creates a new ChromeBrowserMainExtraParts derived class to own
memory management related classes. This includes the new
EnterpriseMemoryLimitPrefObserver, which tracks changes to the
TotalResidentSetLimitMb enterprise policy and starts/stops the
EnterpriseMemoryLimitEvaluator (which the PrefObserver owns) accordingly.
Larger project documentation here:
https://docs.google.com/document/d/1W3FPDyjIAKBcFGNYsHA3EKR1FHrJlbBaqT4_RUnxzq0/edit?ts=5d3f5714#

This also disable the memory::MemoryPressureMonitor as it's not being
used (we're using base::util::MemoryPressureMonitor).

Bug: 980965
Change-Id: Iae632b6681d1574006d38fee9b38bf4d6e5545af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1790290Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705144}