Create SystemMemoryPressureEvaluator from CastMemoryPressureMonitor.

This CL creates the SystemMemoryPressureEvaluator for Chromecast 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: I5fa312fd3519134919aadcfa1e8944a7c932b59a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1758331
Reviewed-by: Sean Topping <seantopping@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
Commit-Queue: Ryan Powell <ryanpow@google.com>
Cr-Commit-Position: refs/heads/master@{#693270}

• base/util/memory_pressure/multi_source_memory_pressure_monitor.cc[diff]
• base/util/memory_pressure/multi_source_memory_pressure_monitor.h[diff]
• chromecast/browser/BUILD.gn[diff]
• chromecast/browser/cast_browser_main_parts.cc[diff]
• chromecast/browser/cast_browser_main_parts.h[diff]
• chromecast/browser/cast_memory_pressure_monitor.h[Deleted - diff]
• chromecast/browser/cast_system_memory_pressure_evaluator.cc[Renamed from chromecast/browser/cast_memory_pressure_monitor.cc - diff]
• chromecast/browser/cast_system_memory_pressure_evaluator.h[Added - diff]