Add the base class for the new memory pressure monitor.

This doesn't do anything yet, it's simply the base classes for the new
memory pressure monitor.

Bug: 771478
Change-Id: I83e65b3128f7eb9634d3ce15a4ef45232a57971c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1518925
Commit-Queue: S├ębastien Marchand <sebmarchand@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642091}
5 files changed