Create classes for Memory Pressure Voting system.

This CL creates the MultiSourceMemoryPressureMonitor,
MemoryPressureVoteAggregator, and MemoryPressureVoter classes described
here: https://docs.google.com/document/d/1W3FPDyjIAKBcFGNYsHA3EKR1FHrJlbBaqT4_RUnxzq0/edit#.
The aggregator class collects votes from voter instances via the OnVote
method. A MemoryPressureVoteAggregator object will be owned by the
MultiSourceMemoryPressureMonitor class, which will be merged with the
MemoryPressureMonitor class after subsequent CL's which will migrate
the OS-specific MemoryPressureMonitors to own a Voter and use that
Voter to inform the Monitor of their votes.

Bug: 980965
Change-Id: Ib84aadca9a4cf74b3c1f8a42787d63a96f5dfd17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1719109
Commit-Queue: Ryan Powell <ryanpow@google.com>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
Cr-Commit-Position: refs/heads/master@{#687282}

• base/util/BUILD.gn[diff]
• base/util/memory_pressure/BUILD.gn[Added - diff]
• base/util/memory_pressure/OWNERS[Added - diff]
• base/util/memory_pressure/memory_pressure_voter.cc[Added - diff]
• base/util/memory_pressure/memory_pressure_voter.h[Added - diff]
• base/util/memory_pressure/memory_pressure_voter_unittest.cc[Added - diff]
• base/util/memory_pressure/multi_source_memory_pressure_monitor.cc[Added - diff]
• base/util/memory_pressure/multi_source_memory_pressure_monitor.h[Added - diff]
• base/util/memory_pressure/multi_source_memory_pressure_monitor_unittest.cc[Added - diff]
• content/browser/BUILD.gn[diff]